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Mike_Mills
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Posted: 4/12/2009 4:54:50 PM
[Last Edit: 4/12/2009 4:56:24 PM by Mike_Mills]
I have a Rock River Arms upper receiver mated to an Armalite lower. I have a Magpul M93B buttstock (carbine length). Operation has been flawless in every way except one.

PROBLEM - The bolt does not lock back the way it should without an empty magazine in the rifle.

CONSIDERATIONS -

1. The bolt locks back just fine with an empty magazine inserted. This is true during manual operation using the charging handle as well as after the last round fired from the magazine.

2. The bolt lock, spring and roll pin have been replaced with brand new RRA parts but this did nothing to fix the problem.

3. If you cycle the bolt manually and pull the bolt lock out at the top (serrated pad), as well as push it in from the bottom, you can usually get the bolt to lock open.

My other rifles work just fine. This rifle is relatively "new", that is it has not been used very much (<1,000 rounds) or used harshly.



THINGS I WAS GOING TO TRY -

1. I was going to degrease the bolt latch and the bottom of the bolt carrier to increase friction between the parts.

2. Swapping bolt carriers between two other rifles did not improve the situation.

3. Replace the buffer and spring with original equipment. I was thinking perhaps the aftermarket spring is too stiff and the latch could not hold it off.

Dano523
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Posted: 4/12/2009 5:26:04 PM
Drop a empty mag in the rifle, pull the charging handle all the way back, and confirm that the bolt catch is rising up in front of the bolt face to lock the bolt back via on the bolt face (not the bottom of the carrier).

If you have the wrong buffer, recoil spring, or even top butt stock screw in the unit (all too long), then the catch will only retain the carrier alone (read bolt face does not retract back past the catch).

Also, the catch holds the bolt via surface tension alone. Bump the rifle, or allow the bolt to retract a faction of an inch reward, and the catch is going to drop out/downward unless the catch is being retained upwards (by an empty mag).
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Mike_Mills
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Posted: 4/12/2009 5:31:15 PM
[Last Edit: 4/12/2009 5:50:55 PM by Mike_Mills]
Originally Posted By Dano523:
Drop a empty mag in the rifle, pull the charging handle all the way back, and confirm that the bolt catch is rising up in front of the bolt face to lock the bolt back via on the bolt face (not the bottom of the carrier).

The latch is catching the bolt carrier.


If you have the wrong buffer, recoil spring, or even top butt stock screw in the unit (all too long), then the catch will only retain the carrier alone (read bolt face does not retract back past the catch).

Also, the catch holds the bolt via surface tension alone. Bump the rifle, or allow the bolt to retract a faction of an inch reward, and the catch is going to drop out/downward unless the catch is being retained upwards (by an empty mag).

This is functioning as you descriibe.







I've been doing some "comparative anatomy" and parts swapping in real time. It turns out, I have a buffer spring that is 12.25" long in the problematic carbine and the carbine that's working fine has a spring that's only 11" long. They both have the same style buffer and both weigh 1280 gr. If I use the smaller spring, everything works fine on either carbine.

I think this is either an extra-power spring or a rifle length spring that's too long. Either way, it seems to be the spring that's the problem, not the other parts.
Mike_Mills
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Posted: 4/12/2009 5:53:29 PM
Dano523, thanks for the help. I put a carbine-length spring (11.25") in and it's working just fine now. It locks back every time now and when it locks, it's catching on the bolt face.

I have a heavy buffer here (1650 gr vs 1280 gr). It is stamped with an "H" on the face. Is there any reason NOT to use that?
fenderfreek
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Posted: 4/12/2009 6:24:56 PM
Nothing wrong with an H buffer. It will just add inertia that decreases cycle rate.
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Mike_Mills
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Posted: 4/13/2009 12:45:47 AM
I used to have a Carrier Weight System in my rifle. It is a weight that fits into the rear of the bolt carrier. It delays the unlocking of the lugs. Will this heavy buffer do the same thing? The CWS is actually very heavy for its size because it's tungsten.