Monday, October 4, 2010

Some Bushmaster ACR Observations

Other ACR Entries
Bushmaster ACR Obervations, Part 2
ACR Review, Part 3

ACR Basic, Photo Borrowed From Bushmaster ACR Manual

I’ve been fiddling with, and doing a little shooting of a Bushmaster Basic ACR. Here are some of the things that I haven’t seen mentioned in other reviews:

The Bushmaster is a modern rifle of unique design, borrowing some aspects of designs that proceeded it.

The bolt is much like an AR-15 bolt, and the trigger group is mostly AR-15 compatible, but other than that, there’s not that much in common with an AR-15. The upper and lower receivers are held together with AR-15 type captive pins; okay that’s one more thing in common.

Geissele SSA and JP AR-15 triggers have been successfully installed into ACR trigger packs.

The ACR bolt carrier rides on rails, sort of like the bolt carrier of a Kalashnikov variant rifle. The gas piston system is more like a FAL.

The ACR’s stock and handguards can be removed by pushing their own take-down pins.  Unlike a HK G3 or MP5-type firearm, the pins are captive.

Even the plastic handguard of the ACR basic model leaves the barrel free-floating.  Unlike an AK or FAL, there’s really no contact at all between the barrel and upper receivers, except where the two parts are coupled at the trunnion.  The gas piston of the ACR is shrouded by the upper receiver, but they don’t seem to touch.  The gas piston is not otherwise housed in a tube or anything.

ACR Barrel/Gas Piston Assembly (Borrowed From Bushmaster ACR Manual

Front End Details

The barrel is pretty tightly fitted to the trunnion. The barrel nut is pretty easy to remove with the attached lever, but actually getting the barrel out of the upper takes a little bit of wiggling.  The manual says to depress the barrel plunger, but doesn't show or tell you what that is.  See picture above.

The gas piston is a little hard to remove.  When you remove the gas plug, it doesn’t pop out, and there’s nothing to grab hold of.  Removing the handguard doesn’t help.  The piston has to be pushed out of the upper with a cleaning rod from the breech end. That, or you could remove the whole barrel assembly from the upper.

The ACR’s charging handle is an odd duck.  To be honest, I haven’t figured out how it works yet.  When the charging handle is locked forward, it is disconnected from the bolt carrier, and that’s what makes it non-reciprocating.  If you pull back on that charging handle, it connects with the bolt carrier.  If you pull back that charging handle, when trying to field strip the rifle, you won’t be able to remove the bolt carrier.  The two parts are connected, and you’ll have to lock the handle forward, and hook the carrier with your finger from the bottom of the upper, and then pull it out.

ACR Bolt Group

The new-production ACR that I’ve been fondling has a solid steel pin retaining the firing pin and firing pin return spring.  The ACR’s firing pin retainer is held in place with an o-ring.

The bolt cam pin is an ACR specific part that is fitted into the left side of the bolt carrier.  The cam only goes into the bolt one way, so you can’t install the bolt into the carrier the wrong way accidentally.  The cam pin does not quite pass all the way through the bolt.

Bolt Comparison

The head of the ACR bolt is almost identical to an AR-15 bolt.  I didn’t leave the AR-15 bolt to make any kind of point, I just haven’t gotten around to cleaning it yet.

Currently, ACRs are only available with a rifling twist of 1 turn in 9 inches.  “Common knowledge” says that a 1:9” twist will not stabilize 75 and 77 grain bullets, however I’ve heard from several ACR owners that these heavier bullets print decent groups.  Jeff Quinn got a sub-MOA group from Buffalo Bore 77 grain Sniper HP ammo with the ACR he tested for GunBlast.  I intend to find out myself, but haven’t gotten that far yet.

There have been some complaints that ACRs won’t lock back after the last round with certain types of magazines.  I’ve tried a D&H aluminum GI mag, a Lancer L5 30-round magazine, one of those Israeli countdown mags, and a 20-round PMag.  I even tried a new-revision PMag with windows that won’t reliably lock back the bolt carrier of either of my AR-15s.  I tried the stubborn PMag two or three times.  I’ve not yet had any problems at all, although the round count is still pretty low.

The safety (selector) easily snicks on and off.  The bolt catch however, gives no leverage assist, and is still stiff on the new rifle I’ve been working with.

There have been complaints that the support-side safety (selector) lever hits the trigger finger of some users when switching the safety off.  It really doesn’t bother me that much.  I trimmed the support-side lever of the ambi safety on my AR (not shown in picture linked below), but the ACR safety I would leave alone.  The ACR safety levers are pretty small, and made of plastic, and I’m not sure how much you can do with them anyway.  I’ve seen military ACRs with no safety lever on the right-side.  I expect there to be a cap or smaller lever available to replace the right-side lever available from Bushmaster or Magpul at some time in the future.

There’s been some comments on the relative recoil of the ACR.  I went from shooting my LMT/Stag AR-15 to the Basic ACR.  My AR has a 16” M4 barrel with carbine length gas system, standard M-16 bolt carrier, an A2 flash hider, and an H buffer.  I can’t say that I noticed any significant difference in recoil.  I do know that a 7.62x39mm AK has more recoil that my AR, and a 5.45x39mm AK has less recoil, but there’s just not that much difference between my AR and the ACR.  I expected the piston-op ACR to have a little more recoil, but that doesn’t seem to be.

Official Bushmaster ACR Home Page

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