Bushmaster Acceptance Inspection (8/07)

AR/M4 AR Technical Discussion
1

Highlights from an initial acceptance inspection of an officer’s Bushmaster carbine. Photos included below are of anomalous items, or items of particular common interest. Photos copyright by me, all rights reserved.

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Manufacturer: Bushmaster
Type: Carbine
Model: Undetermined (Pending)
Serial Number: L452###

Lower Receiver:

 L-prefix serial number. Standard single stage semi-auto fire control group. Note seam on hammer and possible mold mark on left side of hammer. Mil dim receiver, hammer, pivot, and takedown pins. Drain hole in magazine fence. Hex bolt in grip screw.

Buttstock Group:

 Common Bushmaster type. Commercial dim receiver extension. Circumferentially larger and longer (3/4") than mil dim. 6-position. Hole spacing uneven. End of extension cut at angle. Buttstock plastic. Form of current mil type . Loose fit. Receiver extension (castle) nut unstaked. Nut could be unscrewed without tools after two attempts. Standard carbine buffer, correct weight.

Upper Receiver:

 Flat top. Eotech 553 mounted. BUIS is flip-up, Bushmaster OEM, retained by allen screws. No addressing on rails. Extended feedramp machined pre-anodizing.

Bolt Carrier Group:

 Bolt bears “CM” engraving. Extractor spring assembly 4-coil spring with blue insert. Carrier key bears small rectangular indents for staking with no material intrusion on carrier key screws. Bolt carrier includes shrouded firing pin and semi-auto rear tang.

Barrel Assembly:

 16” barrel, marked “B MP 5.56 NATO 1/9”. Chrome lined chamber and bore. M4-type step on barrel. A2-type flash hider installed and timed correctly. Gas block shaved style with Bushmaster OEM flip-up front sight installed over the top of the gas block. Sight retained by allen screws. Gas block and pins installed prior to parkerizing. Sling swivel installed with rivet. Barrel extension cut for extended feedramps prior to parkerizing. Handguards M4-dim, lightweight plastic, single pair black heatshields.

Gauging:

 Passes pertinent gauges.

Other:

 Michiguns Ltd. 5.56mm NATO chamber applied to chamber. Small amount of material removed from chamber.

Function Testing / Live Fire:

 Pending

Required corrections:

 Stake carrier key screws. Stake receiver extension nut to receiver end-plate. Replace OEM extractor spring assembly with carbine extractor spring assembly. Replace grip screw with slotted screw. Apply thread locking compound to all sight components supporting optic. Corrections for Eotech optic as per standing order concerning same.

Recommendations:

 Conditional approval. Return for inspection at every 500 round and 60 day interval(s).

END
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2

Those kind of look like M4 feedramps in there.:confused: :eek:

3

You’re right. The only Bushmaster upper receiver I’ve ever handled that had M4 feed ramps was a Bushmaster piston upper.

4

Ned posted his reamer specs over on a another site and his neck reamer runs a tad larger than even NATO specs. It has a longer freebore dimension (.0750 total length compared to .0500, .0566 standards) and it runs on the high side of the freebore diameter tolerance (.227). I know it’s hard to characterize what constitutes “a small amount of material” but it’s very possible the BM chamber was cut properly to NATO dimensions and the reamer removed a tad more. No real way to tell though.

Good writeup and good pics. While it’s not terribly comforting seeing the flaws in the product, luckily they are easily correctable.

5

Why replace the bolt? Other than having the incorrect extractor spring and insert, what’s wrong with it?

6

My assumption is because it is not MPI

7

Do I see CM on that bolt? If so, CMT bolt, likely is MPI.

8

Bushmaster has publicly stated that they “batch test” bolts, meaning that they “test” one of a group. In other words, they do not test every bolt (and another shortcut taken)
If you have more accurate information as to their testing of bolts, please share it.

9

Pat, you are correct.

Out of interest, BM do in fact check the chrome plating on every barrel with a bore scope for what it’s worth.

10

They are indeed. In my experience, however, they’re the exception not the rule. They have been seen on the Texas DPS guns, some NFA guns for gov end-users, and other special contract guns. Not the norm, esp for the commercial side.

I would still consider your chart accurate, Rob.

11

Thanks.

These flaws would be easy to correct on assembly, which is most befuddling. This is deliberate indifference, or gross incompetence.

12

Quite right, Pat. Just in case.

13

Sorry the pics are bad this time. The bolt marking is “CM.” I’m not sure what that marking denotes. Is it in fact a CMT bolt?

14

Stag bolts have CM stamped into them. I wonder if BM cannot produce enough bolts and are having to buy them from other sources?

C4

15

Seems like it in this case. I know Continental Machine began stamping their bolts with CM stamp this year. I thought I noticed it in the pics. For whatever reason, it seems BM sourced this bolt from CMT. It’s my understanding that CMT MPIs their bolts, I don’t know what BM requests or pays for when they outsource, or if this one is MPI. BM would know though?

Good bolt anyway. A reliability kit and off to the races.

Regards,
Walt

16

SA does MP test each bolt. The question is if BM ordered them that way. One of the things that freaks me out about these large manufacturers is that they will buy just about anything in order to push guns out the door. So some months they use CMT bolts. The next month they get them from Lord knows where. :mad:

C4

17

I suspect indifferenbce. After absorbing all the anecdotes of BM flaws it appears to me BM doesn’t feel these steps are required. They’re obviously putting zero effort into these areas, it’s as if managment is directing the line assemblers to assemble them in the manner that we see on our end. I don’t beleive these are errors or oversights, it’s simply disregard.

I sent my carrier off to Grant months ago to properly stake, done.

I added the HD extractor spring and black insert, done.

I run an A2 stock so castle nut staking is N/A.

My FSB is installed properly, no problem zeroing for windage with either the original RCH or a LaRue BUIS. When I switched to the LaRue, I did have to purchase the .040" taller front site post to get it to zero for elevation without having the post backed almost completely out.

No problems to date with any BCG or FCG parts. I don’t know if they are cast, forged, whatever, but they’ve worked fine.

I’m guessing 2,000 - 2,500 total rounds fired.

I haven’t made any attempt to verify my chmaber dimensions, but I had no problems going through my initial case of XM193 and some small quatities of Win WB. All other loads have been reloads loaded to no greater than published maximums for .223 data, most of it though is backed off anywhere from half a grain to a full grain. No problems with popping primers or case separations or any other typical ammo anomallies.

I’m OK with my BM, but my next build will be from a different mfg.

18

Grant,

Do you have any idea how mnay BCG manufactureres exist? Are there a slew of them or just a small number who end up supplying various weapon mfg’s?

19

I really don’t know for sure. I can think of 4 off the top of my head (not counting BCM as he uses CMT bolts and carriers). BM also makes their own bolts in house and to the best of my knowledge has the most failures of ANY manufacturer.

Honestly, the only bolts I would stay away from are the bolts that BM makes in house and the DPMS bolt as they appear to use some kind of molded extractor. :eek:

C4

20

Sounds like the Bolt can stay.