Ruger mpr - failure to feed

Coworker got a ruger mpr that ran fine the first time to the range

On the 2nd time the bolt was not going back far enough to strip the next round, so it would dent the round and get jammed.

The gas key was ok, the fsb was ok....no leaks

The RO helped him trouble shoot. Tried different ammo, different mags, my spare lower, my spare upper and my spare bcg. They came to the conclusion that the bcg was the issue (and stacking tolerances). His bcg would work in my gun but not in his. My bcm bcg worked in both of our guns. The ruger uses a different bcg (unshrouded). I’m guessing the unshrouded bcg has more drag than a shrouded bcg as goes back over the hammer

My coworker noticed that my gun (bcm upper, Anderson lower with psa lpk) seemed easier to run the charging handle compared to his gun. The latter part of the pull was slightly harder. (I think no mag was installed and the hammer was cocked). I’m thinking the buffer spring is slightly stiffer than an in spec spring

I’m thinking the gun has tolerance stacking issues. When new it ran ok....maybe just barely. I’m thinking the chamber is a little out of spec and and wasn’t cleaned enough so extraction was harder the 2nd time it was used.

My coworker did mention the gas rings were pretty filthy. Ar’s are generally filthy so I don’t think that was an issue

I’m guessing the issues with the gun are an out of spec chamber, out of spec buffer spring and the different bcg cause the bolt slow down enough to cause the failure to feed.

I’m also guessing the rifle gas on an 18” barrel (lower pressure) combined with a carbine tube, carbine spring and carbine buffer (stiffer than a rifle tube set up right?) don’t help

My co worker already got a bcm bcg and will be testing his gun this weekend.

I’m thinking if he got a lighter buffer spring (tubbs?) or a lighter buffer (kac $$$)

Edited to add:
My upper is a 14.5” mid length with a carbine buffer if that matters

Is he running it wet? I would try that for a couple of hundred rounds and see if it smooths out.

Field strip, clean and lube. Lube the gas rings. Sounds like the rifle is dry. Perform a lock back check. If it fails the lock back check, start looking for gas leaks and try good ammo. If a weaker than standard spring is needed, or if an H buffer is heavy enough to cause short stroking, something is wrong. Carbine buffers are just too light, even if the AR is gassed correctly.

Quote:

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Originally Posted by tim808

I’m also guessing the rifle gas on an 18” barrel (lower pressure) combined with a carbine tube, carbine spring and carbine buffer (stiffer than a rifle tube set up right?) don’t help

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A rifle length gas system is fine with a carbine receiver extension, buffer and spring.
There is a sticky troubleshooting post at the top of the page, I would recommend starting there.
There is always the option of sending it back to Ruger for repair.

Andy

Quote:

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Originally Posted by MistWolf

Field strip, clean and lube. Lube the gas rings. Sounds like the rifle is dry. Perform a lock back check. If it fails the lock back check, start looking for gas leaks and try good ammo. If a weaker than standard spring is needed, or if an H buffer is heavy enough to cause short stroking, something is wrong. Carbine buffers are just too light, even if the AR is gassed correctly.

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^^^ All of this. My first thought was a gas leak somewhere, but it suppose it may just be a dry gun that needs to be broken in. The action on Ruger ARs (at least the standard AR556 models) tends to be pretty rough out of the box and the really need to be lubed well for a while, at least in my experience.

After the gas key I would've checked the gas block and tube to be sure they were still installed correctly and weren't obstructed. The handguard on the MPR should come off fairly easily with just an allen key. I would be very surprised if the buffer and/or spring is the issue.


And as Andy said, he can always just send it off to Ruger. Their customer service is usually on top of things and turns most guns around in a couple of weeks, sometimes sooner.

Running it dry -
The RO and I both checked his bcg and neither of us thought it was dry.

My coworker didn’t lube it on the first outing and it jammed. A RO had to help him loosen the bolt and I doused his gun with clp. It ran fine for the rest of the first range visit. My co-worker is a smart guy (mech engineer) and I think he learned the importance of a wet ar. His gun seemed well lubed on the second trip to the range.

Also, when the RO was trouble shooting, I think they also used the ruger bcg in my bcm upper and it worked. So I feel lube was not an issue.

Thanks for the advice on doing a lockback check and the troubleshooting sticky, I’ll pass it on to my coworker.

I still feel there is a tolerance stacking issue since his mpr runs with my bcm bcg and my bcm runs with the ruger bcg. The ruger mpr and ruger bcg should get better (smoother) if they can be shot together a couple of hundred more rounds (as a single shot ar)

Thanks again for all the help!!

Edited to add:
I’m going to bring a spare buffer spring and carbine buffer the next range visit and see if that makes a difference.

Some kind of pics and some more info about specs would be great.

My friend is now following this thread. I’ll ask to him to register and post info/pics

Tnx IG