Tuesday, March 3, 2009

It's not pretty, but it works.

During my last range session with the 9mm AR-15 built with a Bushmaster lower receiver and Rock River Arms 9mm top end, the bolt failed to go fully forward a few times.

This problem grew worse as the bolt grew more fouled, which can be a problem with carbines chambered for pistol cartridges. At times the bolt wouldn't have enough momentum to strip cartridges off the top of the magazine. Sometimes an empty would not completely extract and blocked the bolt's travel. I took the bolt out, wiped it down on a leg of my faded, holey, blue jeans, and noticed an improvement. However, every so often I'd have to slap the upper receiver opposite the ejection port to jar the bolt home.

Colt 9mm buffer assembly, 5.2 oz.

I am running a Hahn Precision dedicated magazine well block that uses the standard 5.56 bolt catch, a standard carbine spring and a Colt 9mm buffer. The Colt buffer, as well as the RRA 9mm equivalent or DPMS counterweight buffer used in pistol-caliber rigs, allows the bolt to travel far more than it needs to for a 9mm. The maximum overall length of a .223 round is 2.26 inches. The max overall length for 9x19mm ammunition is 1.169 inches. Because the bolt travels too far back, I've had live or ejected rounds trapped between the fully retracted bolt and the ejector.

So I decided to buy a spacer to place in the far end of the buffer tube to limit the bolt's travel. However the one I first settled on would have cost me more than $30 including shipping, which was more than I paid for the Colt buffer assembly.

The frugal gene inherited from the Scot ancestors kicked in, and I dug a prime bolt out of a bin. I rounded the hex bolt head and turned it down to .980 diameter. I also turned the shaft down to .78 diameter after cutting the bolt to an overall length of 1.08 inches. I thought that length, .011 in. shorter than the difference between the two cartridges' overall lengths would be close.

Colt buffer assembly, left, and spacer fitted to CAR action spring, right

The opposite end of the carbine action spring fit the bolt shaft perfectly. The overall diameter of the spacer allowed it to slide down the CAR-length buffer tube. However, it fit tight enough to not allow side-to-side rattles. However, I had to cut the overall length down to .906 so the bolt stop would engage.

After the dimensions were set, I polished the piece, gave it four baths with cold blue, oiled it and set it aside to cure. What little testing I've done has been flawless. I'll put the carbine to a more extensive test Thursday.

The bolt does close with a bit more snap despite not traveling as far. The end of the recoil spring sits .54 inch, the thickness of the spacer's base, forward of its former resting point at the end of the tube. Because the 9mm carbine is blowback operated, the slight boost in spring power will push a little more gas out the muzzle and less fouling will result. It also takes less effort to chamber a round by working the charging latch.

Don't try doing this at home if you're already running high-pressure ammunition such as NATO M882 9mm ball, +P, +P+, or your home-rolled zombie stoppers. Go slow, don't blow stuff up and rearrange your body parts.

Hahn Precision sub caliber buffer assembly, 8 oz.--note the longer full-diameter section that increases action spring tension.

If I were to build another 9mm carbine, I'll probably try Hahn Precision's sub caliber buffer. It is longer, heavier at 8 oz., and has the thicker base to increase spring tension. It does all the stuff my 5.2 oz. Colt buffer and homemade spacer does. Despite costing twice what I paid for the Colt buffer, a bolt and cold blue, it is a whole less trouble.
UPDATE: I shortened the spacer, reducing the overall length to .840 inch after more range work Thursday afternoon. The bolt catch would sometimes fail to activate before the spacer was shortened a bit more. The carbine is running 100 percent now. The empties land in a much more consistent and tight pile nearly eight feet out at 2 to 3 o'clock from the ejection port. Also, the extractor hook isn't scarring the case rims near as much.


Brigid said...

I returned from my road trip to find the 8 mm mauser casings you sent. Thanks!!!. Those will be put to good use. Best to the Mrs and the family.


Somerled said...

You're welcome and it's good to know you are home safe once more. My family is doing well--my greatest blessing. I enjoy reading about your family, Dr.

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