mini-14 - ac556

The Small Arms Review - Vol. 7 No. 12 - September, 200418

Visit us on line at: www.smallarmsreview.com

The author bought a Mini-14 soonafter its introduction and immediately in-stalled a variable 3x-to-9x optical sight.He still owns it and it has served him well.Its five shot group accuracy, with militaryball ammunition, is on the order of 1.5inches at one hundred yards. Serious rifle-men sneer at this degree of accuracy ofcourse. Remember though, this is an out-of-the-box semiautomatic rifle, using less-than-the-best ammunition and it has metthe author’s needs for thirty-plus years. Re-liability-wise, nothing could have been bet-ter than his Mini-14. When using a genu-ine Ruger manufactured magazine, theauthor has never experienced a failure withit.

A fully automatic version of the RugerMini-14 was also introduced. The basicblue-steel model was called the AC-556.Another model was available that accepteda standard M7 bayonet and was designatedAC-556GB (“GB” for government bayo-net).

A very compact version of the AC-556was soon added to the Ruger product line.

It has been over thirty years since Sturm, Ruger, & Com-pany introduced a semiautomatic 5.56x45mm (5.56 NATO)carbine. It looked like a small version of a US Army M14and it was, in fact, called a “Mini-14.” The Ruger carbinewas an immediate sales success and it has been in continu-ous production since its introduction.

It had a thirteen inch barrel and a side-fold-ing stock. This model was called the AC-556K.” It is assumed that the “K” in thecompact version’s designation stands forthe German word “Kurtz,” meaning“short.” If a 13-inch barrel AC-556K isplaced into a standard AC-556 stock, theresult is a very short select-fire carbine thatpoints very naturally.

Before long, AC-556s became avail-able in stainless steel versions. The prefix“K” was added to the designation of thosethat were made of stainless steel and thusbecame the KAC-556 and KAC-556K ifit was the short barreled version with side-folding stock.

In addition to their fire control selec-tors, full-auto Ruger carbines had flash hid-ers that served a dual purpose. Besides theobvious function of shielding muzzle flash,it also served as a rifle grenade launchingfixture. When launching a rifle grenade, aspecial type of blank cartridge is used topropel the grenade.

The safety on the full-auto AC-556Ruger carbine is ergonomically locatedwithin the trigger guard. This is the samelocation as found on the M1 Garand andthe M14.

The fire control selector on the AC-556, like the M14’s, is located above thetrigger on the right rear side of the receiver.

Top, the author’s thirty-year-old Ruger Mini-14. It has not malfunctioned in thirty-plusyears. Bottom, a Ruger KAC-556GB.

The Small Arms Review - Vol. 7 No. 12 - September, 2004 19


The selector has a locking button on topthat assures that the AC-556 remains in theuser-selected mode of fire. The button mustbe pressed downward in order to reposi-tion the selector.

Unlike an M14, an AC-556 fire con-trol selector has three firing modes: semi-auto (straight up), three-round-burst (mid-position) and full-auto (horizontal). Thesemi-auto and full-auto selector positionsare self explanatory. The three-round-burstposition needs some further discussionthough. Due to the way in which it func-tions, when the burst is first selected, itdoes not always produce a three-roundburst. Depending on the circumstances, theselector can produce bursts of one, two orthree rounds on the first pull of the trigger.This is not generally a problem but an ex-planation is in order.

When an AC-556 is fired in eithersemi, burst or full-auto, a pawl engages andadvances a ratcheting round-counter.When burst is selected and the round-counter reaches its third step, the AC-556disconnector is engaged to prevent anothershot. The explanation for the uncertainburst becomes clear upon reflection.Though the round-counter is disengagedfrom the disconnector when either semi-auto or full-auto is selected, the round-counting pawl is never disengaged fromthe trigger. It counts the fired rounds in thesemi-auto and the full-auto modes as well.As a result, when burst is engaged, theround-counter could be resting on eitherits first, second or third step. The first pullof the trigger after burst is selected could

produce a burst of one, two or three rounds.After firing the first burst in the burst mode,the round-counter will become properlysynchronized. All subsequent bursts willthen be of three rounds unless the triggeris released before the disconnector endsthe burst.

At the AC-556’s relatively slow cy-clic rate of 750 rounds per minute (rpm),with just a little practice a shooter can setthe selector to full-auto and fire two, threeor four founds at will. Few experiencedAC-556 shooters use the burst position ofthe selector.

Magazines

Genuine Ruger AC-556 magazines areproduced in capacities of five, twenty andthirty rounds. Like the detachable boxmagazines of most modern rifles, AC-556magazines are of the staggered column,left/right feeding design. They are veryreliable and easy to load.

Other manufacturers have producedmagazines for the AC-556. Some of thesehave proven to be just as reliable as genu-ine Ruger magazines. Others have beenless reliable. In the author’s experience,magazines that were made for the AC-556by Ramline and Thermold have proventhemselves to be nearly equal to genuineRuger magazines.

Two 90-round magazines have beenmade for the AC-556. The one made byM.W.G. is nothing more than a very longstaggered column magazine that has beentwisted into a left-offset circle. Loading it

begins easily but it becomes difficult afterinserting approximately fifty rounds. TheM.W.G. 90-round magazine seems to havemore internal friction than other staggeredcolumn, left/right feeding magazines. Thisis evidenced by the AC-556 operatingsprings’ lack of sufficient force to reliablystrip the first ten rounds from a fully loadedM.G.W. magazine. For the first ten rounds,the AC-556 bolt stops with a round only

Right: Three Ruger AC-556 fire control selectors. The top one is set to semi-auto. The middle one is set to three-round-burst. The bottom one is set tofull-auto.

Below: From left to right are: An M.W.G. 90-round drum magazine; an AK47 90-round drum that was converted prior tothe HICAP ban for use in an AC-556; a Thermold 30/40-round AC-556 magazine; A Thermold 30-round AC-556 magazine;A clear plastic Ramline AC-556 magazine; Ruger 30, 20 and 5-round AC-556 magazines.



partially pushed into its chamber. To con-tinue firing, it is necessary to manuallypush the bolt closed. During the next tenrounds, occasional failures of this typecontinue to occur. No failures of theM.W.G. magazine were experienced if onlyseventy rounds were loaded into it. Con-sidering the difficulty of loading the lasttwenty rounds, even if the M.G.W. maga-zine fed rounds 71-90 reliably, most shoot-ers would probably elect to load only sev-enty.

More evidence of the M.W.G. 90-round magazine’s high internal friction isthe slower AC-556 cyclic rate while usingit. When using a Ruger 20-round maga-zine, the AC-556 cycles at 750 rounds perminute (rpm). When the M.W.G. magazineis loaded with seventy rounds, the initialcyclic rate is only 500 rpm. The cyclic rateincreases to 700 rpm as it fires the lastthirty rounds in the M.W.G. 90-roundmagazine.

The M.W.G. magazine is asymmetri-

cal. This causes a little unbalance but it isnot a problem for a right-handed shooter.The left-side offset magazine is cradledand supported in the shooter’s left armwhen firing in the standing-position.

Another manufacturer made a 90-round magazine for the AC-556 by graft-ing the upper portion of a Ruger magazineto an AK47-type drum magazine. The staffportion of this magazine is centrally lo-cated causing the AC-556 to balance well.

Like the drum magazines of Suomis,Thompsons and PPSh41s, the ammunitionin the AK47/AC-556 drum is stacked in along, circular, single column. All other AC-556 magazines are of the two column type.

An AK47/AC-556 drum loads andfunctions like a Thompson drum magazine.Whereas a Suomi or a PPSh41 drum iswound before it is loaded, an AK47/AC-556 and a Thompson drum magazine iswound after it is loaded. In theory, this isan advantage as a drum magazine of thistype may be stored loaded but unwound

for long periods without stressing themagazine spring.

Like the M.W.G. 90-round magazine,the AK47/AC-556 drum magazine hasmore internal friction than a genuine Ruger20-round magazine. When using theAK47/AC-556 90-round drum magazine,the AC-556 cyclic rate is slowed to 650rpm but, unlike the M.G.W. 90-roundmagazine, the cyclic rate is nearly constant.

The author was unable to wind hisAK47/AC-556 drum magazine’s operatingspring sufficiently to permit it to fire all90 rounds. After firing about 50 rounds,the AC-556 would stop with its bolt closedon an empty chamber. This condition in-dicates that the drum magazine’s operat-ing spring is unable to elevate a fresh roundbefore the fast-cycling bolt closes. Whenthis occurs, firing can be resumed by re-winding the magazine spring and recyclingthe bolt.

7.62x39mm Conversion

The semi-automatic Ruger Mini-14 isalso offered for sale in the Soviet AK47caliber of 7.62x39mm. Supplies of this am-munition have been plentiful and cheap.The full automatic AC-556 is not manu-factured in this caliber though. Can one beconverted to 7.62x39mm caliber? Yes, itis possible but there are considerations thatmake this conversion impractical.

Ruger will sell neither a 7.62x39mmbarrel nor a bolt unless they also install itin the firearm themselves. More impor-tantly, Ruger will not make this caliber con-version for an AC-556 owner. For thosewho are determined to have an AC-556 in7.62x39mm, the most practical way is to

Above: Two 90-round magazines and 90-rounds of loose ammo. The AK47/AC-556 magazine is on the right and theM.W.G. magazine is on the left.

An assembled (left) and disassembled (right) gas block.



buy a Mini-14 in this caliber in order toacquire the needed parts. The skills of acompetent gunsmith would be required toinstall the barrel.

There is also a very practical reasonfor not converting an AC-556 to5.56x39mm. To avoid the complicationsthat were caused by the onerous provisionsof the Magnuson-Moss Act (Public Law93-967), Ruger sells its firearms without awritten warranty. Ruger has a reputationthough for repairing all unmodified Rugerfirearms without charge. Ruger would notrepair a caliber-converted AC-556 eithergratis or for pay. Of course, the BATFwould have to be notified as well of anychanges in barrel length or caliber sincethis is not a “drop-in” and easily removedalteration. An AC-556 owner should giveall this careful consideration before mak-ing a conversion to this caliber.

.22 Rimfire AC-556 Conversion

Due to the 5.56mm NATO round’slong range and deep penetration, extra caremust be taken when selecting a place toshoot. It must have a very good backstopand a large unoccupied safety area locatedbehind it as well. Additionally, when com-pared to a pistol caliber, the 5.56mmNATO round is very loud. For these rea-sons, a backyard shooting range is gener-ally unsuitable for the AC-556. A simplesolution is available though.

Jonathon Arthur Ceiner manufacturesand sells a .22LR conversion kit for theRuger AC-556. With certain brands of highvelocity .22LR ammunition, it functionswell in all three modes of fire. Moreover,the .22LR caliber conversion kit uses the

standard AC-556 barrel. If it is ever nec-essary to return the AC-556 to Ruger forrepair, the Ceiner .22LR conversion kit canbe removed quickly and the original 5.56NATO parts reinstalled.

The .22LR kit is packed in a sturdyplastic storage case. It installs in the AC-556 using only a 5/32 Allen wrench andthe wrench is supplied with the kit.Ceiner’s installation instructions are com-plete and relatively simple though first timeinstallation is a learning experience requir-ing about twenty minutes. Subsequent in-stallations take less than ten.

Ceiner’s AC-556 .22LR conversionkit is priced reasonably at $129. Unfortu-nately, unless the buyer is a law enforce-ment officer, only 10 round magazines areavailable for it at this time due to the

HICAP magazine ban. If the HICAP banis allowed to sunset, the 30-round maga-zine will once again be available. Fortu-nately, the author bought his .22LR con-version kit and five extra 30-round maga-zines long before the HICAP ban was en-acted.

The AC-556 cyclic rate with .22 CCIMini-Mag ammunition was 1,340 rpm. Atthis fast rate of fire, the three-round burstfeature proved to be very useful. When fir-ing at 25 yards (23 meters) with three-round bursts, the .22LR AC-556 was verycontrollable. Most of the hits were con-tained within a 3.5 inch (89 mm) circle.

Initially, the Ceiner .22LR conversionkit functioned very well. After about 2,000rounds though, stoppages began occurring.In nearly every stoppage, the round struck

the face of the barrelslightly above the chamber.These failures occurredwith all five 30-roundmagazines. A thoroughcleaning was no help.

As good fortune wouldhave it, the author had one

Cleaning the orifices in the AC-556 gas piston with a 1/16and 1/8 inch drill bits. The small gas port bushing (upperright) is cleaned with a 1/16 inch drill bit.

Left: When reassem-bling an AC-556, makecertain the “linkpositioner assembly”(the part with the U-shaped notch at arrows)is in its most vertical po-sition. The assembly onthe right is correct.



remaining 30-round .22LR magazine thathad never been used. It was the one thatwas originally supplied with the Ceiner.22LR kit and it had never been removedfrom the storage case. With this magazine,the .22LR AC-556 functioned again withnear perfection.

A dial caliper was used to comparethe one functioning magazine with the fivenow unreliable ones. The only major dif-ference between them was the gap betweenthe new and well-used magazines’ feedinglips. The functioning magazine’s feedinglip gap measured 0.17 inch (4.32 mm). Thesame measurements on the non-function-ing magazines were all greater than 0.21inch (5.33 mm).

The magazine follower was removedbefore adjusting the feeding lip gap. A vicewas then used to carefully close the feed-ing lip gaps of the unreliable magazines.It was found that the well-used magazineswould function again when their lip gapswere set in the range of 0.16 inch (4.1 mm)and 0.20 inch (5.1 mm).

To achieve proper functioning, it is

necessary for the feed lips to be parallelwith a constant gap width. For this reason,avoid using pliers or a hammer to closethe gap.

Further testing revealed that the .22LRmagazine lip gap problem can be avoidedentirely if the 30-round magazine is onlyloaded with 25 rounds. Loading the lastfive rounds into the magazine requiresconsiderably more effort than loading theprevious twenty-five. The extra tension,caused by the last five rounds, seems to bethe culprit in spreading the magazine lips.

Bolt Bounce

All .22LR semi-autos fire from an un-locked breech. The mass of the bolt andthe resistance of the operating spring keepthe bolt closed during the high-pressureportion of the firing cycle. After the bulletexits the barrel, the residual gas pressurepushes the bolt rearward and compressesthe operating spring. The compressed op-erating spring then pushes the bolt forward,stripping a fresh round from the magazine

and driving it into the chamber. When therapidly moving bolt strikes the face of thebreech, it bounces back a short distance.The pressure of the compressed operatingspring restrains the bolt and quickly pushesit back into battery.

Bolt bounce occurs in all semi-auto.22LR firearms. While it is not a problemin semiautomatic fire, it is a problem infull automatic fire. In a semi-auto, the boltbounces out of battery and is then pushedback into battery before the semi-autoshooter can release the trigger and pull itagain. On a full-auto .22LR firearm though,the sear releases the hammer as the boltreaches battery. If nothing prevents the boltfrom bouncing back, the hammer willstrike the bolt as it bounces out of battery.A misfire often occurs because much ofthe falling bolt’s energy is absorbed whenit strikes the out-of-battery bolt.

All reliable closed-bolt .22LR ma-chine guns, except the Ceiner .22LR/AC-556, have a moving anti-bounce weight lo-cated on the bolt. A microsecond after thebolt strikes the breech; the moving weightstrikes the bolt and absorbs the rebound-ing bolt’s energy, thus preventing thebounce.

There is no obvious explanation as towhy the Ceiner AC-556 .22LR conversionkit works so well without an anti-bounceweight. The answer may be the AC-556’smassive coil hammer spring. It is muchstronger than the hammer spring of anM16. The AC-556 hammer spring seemsto overcome bolt-bounce with brute force.It appears to push the bouncing bolt backinto battery while retaining sufficient en-ergy to crush the priming compound in thecartridge rim.

A disassembled KAC-556GB



A New Problem is Discovered

After shooting .22LR cartridges in anAC-556, thoroughly clean the chamberbefore firing any 5.56mm NATO rounds.The chamber may have accumulated de-bris that could prevent the bolt from clos-ing properly. Firing a high-pressure5.56mm NATO round out of battery is dan-gerous. Cleaning the AC-556 chamber isnot difficult and an M16 chamber clean-ing brush or a .357 pistol brush work well.

If the AC-556 .22LR kit is shot a lot,the gas port will become blocked with va-porized lead and unburned powder. Thisblockage does not affect the .22LR kit’sfunctioning as the kit does not use the AC-556 gas system. It is self contained andoperates on the “blow back” principle.

After the author removed his .22LRkit and reinstalled the 5.56mm NATOparts, the AC-556 would not function un-less he cycled the bolt like a straight-pullbolt action rifle. Before it would functionagain in semi or full-auto, it was neces-sary to disassemble the AC-556 gas sys-tem and clean it.

Begin by following the AC-556 ownermanual’s disassembly steps. Some RugerAC-556 owners may not have an owner’smanual. Most of the AC-556s that are inprivate hands came from police depart-ments when they traded them in for newerguns. Only rarely were the owners’ manu-als passed along with the firearms in trade.This is not a problem as the Ruger Corpo-ration wants every one to know how to usetheir guns safely. AC-556 owners who lacka manual for their AC-556 can get one free.Write Ruger and politely ask for one andenclose a copy of your Form 4.

When beginning AC-556 disassembly,do not overlook one very important step.Make certain that the selector is set to thevertical, semi-auto position before disas-sembly and do not reposition it until afterthe AC-556 is reassembled. Failing to doso almost guarantees that the “rocker armassembly (MA12600)” and “linkpositioner assembly (MA12700)” will bedamaged.The Ruger AC-556 manual describes thegas system as being, “self-cleaning with5.56mm NATO ammo.” For that reason,the operating manual does not describehow to disassemble or clean the gas sys-tem.

After fieldstripping the AC-556 per

the owner’s manual, use the following in-structions to clean the gas system. Referto the picture on page 5 of the owner’smanual as it clearly shows the parts of thegas system.

Use a 9/64 (0.140) inch Allen wrenchto remove the four cap-screws that clampthe top and bottom halves of the gas block(MS03500) to the barrel. Watch carefullyas the lower half is removed from the bar-rel as the small gas port bushing(MS02500) could fall out. If it stays in itsrecess, gently pull it out. After it is re-moved, the gas piston (MS03600) willslide out of the lower half of the gas block.

Using only finger power to turn it,clean the small gas port bushing with a 1/16-inch drill bit. Scrape away the foulingwith the bit being very careful not to en-large the orifice.

The gas piston has two orifices. Cleanthe smaller one with the 1/16 inch drill bit.Clean the larger one with a 1/8-inch drillbit. Again, use finger power only. Justscrape out the fouling.

The gas port in the barrel is smallerthan 1/16-inch. Use an open paper clip toclean it.

Reassemble the gas system. Avoidover-tightening the cap screws. The threadscan be stripped. Run a cleaning rod with apatch through the barrel to remove anyfouling that was pushed into the barrelwhile cleaning the gas port. Clean thechamber as previously described. Finishreassembly per the owner manual instruc-tions. Before replacing the trigger group,be certain that the selector is set verticallyto semi-auto. Also make certain that thevertical arm of the “link positioner assem-



bly” is straight up as well.

Shooting an AC-556

The rear sight of an AC-556 is an ap-erture that is adjustable for both windageand elevation. Unlike an M16 rear sightthough, an AC-556 rear sight is not aquickly adjustable flip-over “L” that iszeroed for two distances. Given the flatshooting trajectory of the 5.56mm NATOround, a two-position rear sight is unnec-essary. If an AC-556 rear sight is adjustedso that the bullet hits dead-center at 300

yards (274 meters), the bullet’s trajectorywill not be more than 5 inches (12.7 cm)above or below the line of sight betweenthe muzzle and 350 yards (320 meters).For most users of an AC-556, this is quitesufficient.

Like an M16, an AC-556 is a light-weight full-auto firearm. Neither one isparticularly controllable in full-auto. Bothhave light-weight air-cooled barrels aswell. More than one RKI has reported thatfiring three consecutive 30-round bursts ineither firearm will permanently diminishits accuracy and make the barrel so hot that

if a round is left in the chamber, it will“cook off” in ninety seconds. For thesereasons, sustained full-auto fire is not rec-ommended in either.

Firing several consecutive 30-roundbursts of .22LR ammo will leave lead de-posits in the barrel but the heat that is gen-erated is not sufficient to damage it. Theauthor has never heard a report of a .22LRcartridge “cooking off” in either an AC-556 or in an M16. Safety minded shooterswill not tempt fate though by leaving anunfired round in the chamber of either af-ter firing multiple long bursts.



Ruger AC-556 or Colt M16?

Which full-auto firearm will best suita SAR reader’s needs, a Ruger AC-556 ora Colt M16?

An AC-556 has three minor advan-tages over an M16 and one major one. Iflong exposure to saltwater or lower cost isa requirement, the obvious choice is theRuger AC-556. Stainless steel KAC-556sare not rare and an AC-556 is generallypriced at one-half to one-third of the costof a Colt M16. Except for a very few GroupIndustries stainless steel M16 receivers,stainless steel is not an option for an M16.For most users however, stainless steel isunnecessary.

Third, the AC-556 gas system does notblow unburned powder and other residueof combustion into critical places in thereceiver. Due to the nature of the M16 gassystem, it will become dirtier faster thanan AC-556. An AC-556 will functionlonger without cleaning than a M16. How-ever, for non-military use, fouling accu-mulation is seldom a problem.

Fourth, and a very useful feature, un-like most machine guns, the AC-556 is notreadily identifiable as a machine gun. Thedifferences between an AC-556 and aMini-14 are subtle. Both lack a prominentpistol grip and neither look like the newsmedia’s idea of a dreaded “assault rifle.”This is particularly true if an optical sightis installed. An AC-556 is a wise choiceof firearm if stealth is a requirement. AnAC-556 may be carried, without undue at-tention, anywhere that a hunting rifle maybe taken.

Regarding accuracy, as they arrive in-the-box, neither a Colt M16 nor a RugerAC-556 is a tack driver. Much more de-velopmental work has been done on theM16/AR-15 though than on the AC-556to enhance accuracy. It would be mucheasier to turn an M16 into a minute-of-angle rifle than would an AC-556.

The only practical caliber conversionthat can be made to a Ruger AC-556 is to.22LR. A reliable .22LR kit is available forthe Colt M16 as well. Due to the M16’seasily removable upper receiver though,many other practical caliber conversionsare available for it. These include7.62x39mm, 9mm Parabellum, .45ACPand .50 Action Express. Of the two full-auto guns, the M16 is the most versatile.

Other practical considerations are



spare parts and accessories. Virtually nospare parts or accessories, other than opti-cal sight mounts and magazines, are avail-able for a Ruger AC-556. As for the Colt,few firearms have as many spare parts anddrop-in accessories available to them asdo M16s and AR-15s. The RKIs’ generalconsensus of opinion is this, “The RugerAC-556 is a rifle. The M16/AR-15 is asystem.”

The Author’s Opinion

There are many good reasons to owna Ruger AC-556. Simply not owning one

was a good enough reason in the begin-ning for me to buy one. I rationalized thatthere was an empty space in my gun vault.

There are more practical reasonsthough to buy an AC-556. At one-third toone-half the cost of an M16, an AC-556 ismore affordable. The AC-556 is availablein stainless steel. With Ruger and certainother magazines, it functions flawlessly. AnAC-556 can pass for a sporting rifle andthere is a reliable .22LR kit for it. TheRuger factory will fix an unaltered AC-556if repairs are needed.

Unless the potential owner needs theoption of quick centerfire caliber changes,

the M16 has no advantage over the AC-556. An AC-556 is the cost-effectivechoice.

A reliable .22LR converted AC-556is a near ideal firearm to use when intro-ducing novices to full-auto fun. The ammois cheap, there is virtually no recoil andthe sound of the shot is not intimidating. Itis pretty much guaranteed that smiles andexcitement will be generated when new-comers are exposed to this full-auto funand is sufficient reason to own a Ruger AC-556.

Sources

Dial CalipersDillon Precision Products, Inc.

8009 E. Dillon’s WayScottsdale, AZ 85260

480-948-8009

.22LR Conversion KitsJonathon A. Ceiner, Inc.

8700 Commerce St., Dept SARCape Canaveral, FL 32920

321-868-2200

mini-14 - ac556

Documents