I have an LMT MRP 16" upper on a PWA lower that has a Magpul stock on it.

I'm having some FTEj problems every 5-10 rounds or so, and I suspect a problem with the lower.

Therefore, I'd like to use the full-size stock and lower from my 24" Varminter to diagnose this issue.

Can you give me some idea of what kind of problems (if any) there might be if I switch this out temporarily?  My guess is that the longer spring and buffer may lead to cycling problems.  It's just a bitch to move the Magpul to a different lower, especially after grinding down the shims to get the rotation of the tube just right...  Thanks.

No, it works fine.  Just switch lowers.

Barrel length has no effect on your stock selection, you won't have any problems changing out your Magpul for an A1/A2 stocked lower.  If anything, I'd be willing to bet your cycling problems will disappear...

The basic rule when changing out stocks themselves is to keep the short/CAR sized spring, buffer and extension together and the full sized/rifle spring, buffer and extension together... Mixing and matching buffers, springs and extensions is a bad thing...  

edited for gramma...

I've got an 18" MRP and it runs good.

Can you describe the malfunction?  We might be able to figure out what's wrong.
 

Thanks to you both.  That makes good sense.  It seemed odd to me that barrel length would make any difference, but I'm still learning here.

Mix and matching of springs and buffers... yes, that must have been what I'd seen earlier as being a potential problem, but as long as the right spring/buffer's in the right tube, it shouldn't be an issue (barring other unknown problems).

Quoted: I've got an 18" MRP and it runs good. Can you describe the malfunction?  We might be able to figure out what's wrong.

Zak,

I think it may be my PWA lower and parts kit.  Here's the deal.  (Bear in mind I'm using South African surplus 55-grain ammo from 1986, so I cannot rule that out yet.  Need more ammo).  After about 4-5 rounds, the bolt extracts the spent casing back all the way without ejecting, and the next live round from the mag is partially leaving the mag and crunched underneath the spent casing.  After I clear it (which is easy to do), it will go for another 5-10 rounds before the identical problem re-visits.

What concerned me was that while playing with the setup at home, I noticed that the small lugs on the bottom of the bolt were BARELY contacting the bolt stop.  It was as if I had to push the bolt stop higher so that a good engagement could be made.  It looks as if the mag was not seating and locking high enough in the mag well to push that bolt stop as high as it ought to go.  So, I would bang the mag up from the bottom with my hand every now and then, but I don't want to do that forever.  I actually thought this was a mag problem, but I have the same issue with all the mags (10 of them).  All the mags are NIW Labelle's.

I hate to think it's a lowere receiver out of spec issue, because of it being a PWA (LMT) lower.  But, I'd MUCH rather it be that than my MRP (God forbid).  When I try the other lower out with the MRP, I think the results will be telling, assuming good ammo.

Any wisdom is appreciated.  I will go back to the FAQ's and read up there as well.

Don't be so quick to blame the lower.  I had the same problem with some Labelle mags not pushing up the bolt catch strongly in more than one lower.  New mag springs solved that problem.

My MRP needed a few hundred rounds to break in and run 100%.  It uses an LMT Defender Lower and a Magpul stock as well.  Try an H buffer, and make sure your recoil spring is fresh.

Quoted: Don't be so quick to blame the lower.  I had the same problem with some Labelle mags not pushing up the bolt catch strongly in more than one lower.  New mag springs solved that problem. My MRP needed a few hundred rounds to break in and run 100%.  It uses an LMT Defender Lower and a Magpul stock as well.  Try an H buffer, and make sure your recoil spring is fresh.

Good points.  Only have about 100 rounds through the MRP right now.  I guess I could try some stronger mag springs and see if that helps.  But, I suppose when I try these new Labelle's in my trusty ol' Bushmaster lower (from my Varminter), I'll know real quick whether it's a mag issue.  Carbine buffer and spring are brand new.  Need to educate myself on the H buffer, but I ought to be able to resolve the problems without going there yet.  Thanks for the spring comments; hadn't focused on that.

You might also want to try a stronger extractor spring as well.  Extra power mag and extractor springs can be had from Wolff.  Feeding and cycling problems can usually be solved with the use of a stronger extractor spring and an H buffer.  A lot of places sell H buffers on these boards.

I have no experience with FTE malfs..  mine to date have been feed related.

When I shoot full power XM193 through my MRP, it runs better with a NM-weight carrier (along the same line of thought as the H carrier).    The JP SS carrier runs too fast with XM193, but is fine with BH75gr.

-z

ETA: to clarify, with regard to malfs to date, I was speaking of all my AR's, not the MRP.  Usnig the LBC NM-weight carrier, it runs 100% with anything.  With the lighter-weight JP SS carrier, it FTF's with XM193, but runs fine with BH75gr.

-z

Quoted: I have no experience with FTE malfs..  mine to date have been feed related. When I shoot full power XM193 through my MRP, it runs better with a NM-weight carrier (along the same line of thought as the H carrier).    The JP SS carrier runs too fast with XM193, but is fine with BH75gr. -z

I have had same result (0 FTE or 0 FTF) as Zak, with the exception I have run only 69-75gr Federal and Hornaday loads through my 16" SS MRP (different lowers with VLTOR carbine & LMT/Crane stocks).  With the exception of my 6.8, I run only full Colt BCGs in my 5.56MM ARs. Though,I will be trying a standard LMT BC with Colt bolt shortly (hopefully shortly). Unless I missed it in one of the posts, what BCG are you using in your MRP?

Quoted: Quoted: I have no experience with FTE malfs..  mine to date have been feed related. When I shoot full power XM193 through my MRP, it runs better with a NM-weight carrier (along the same line of thought as the H carrier).    The JP SS carrier runs too fast with XM193, but is fine with BH75gr. -z I have had same result (0 FTE or 0 FTF) as Zak, with the exception I have run only 69-75gr Federal and Hornaday loads through my 16" SS MRP (different lowers with VLTOR carbine & LMT/Crane stocks).  With the exception of my 6.8, I run only full Colt BCGs in my 5.56MM ARs. Though,I will be trying a standard LMT BC with Colt bolt shortly (hopefully shortly). Unless I missed it in one of the posts, what BCG are you using in your MRP?

I hadn't mentioned the BCG, but it's the LMT standard carrier and with enhanced bolt.  Getting very curious about possibly swapping the BCG between my two AR's to experiment on this.... may learn something new...

I am running an LMT Enhanced bolt in my MRP, no problems with the bolt here.  

Does the ejector (not the extractor) have tension?

If you chamber a round, eject the mag, and fire the round (ie, firing with no mag), does it eject every time?

-z

Quoted: I am running an LMT Enhanced bolt in my MRP, no problems with the bolt here.   Does the ejector (not the extractor) have tension? If you chamber a round, eject the mag, and fire the round (ie, firing with no mag), does it eject every time?

Another good suggestion.  I'm gonna print this thread and take it (and a camera) with me to the range and conduct some controlled experiments.  I can see that 90% of the battle is correctly diagnosing the situation to isolate the problem.  I'll report back after the weekend with pics.  Might as well make this fun.

Zak, thanks also for the tip a while back on using the AK tool for the Troy front sight post.

Quoted: Another good suggestion.  I'm gonna print this thread and take it (and a camera) with me to the range and conduct some controlled experiments.  I can see that 90% of the battle is correctly diagnosing the situation to isolate the problem.  I'll report back after the weekend with pics.  Might as well make this fun.

That's a good idea.  Feel free to swap around bolts, carriers, and entire bolt carrier groups for testing.. the only caveat there is that don't use a really old bolt in the new MRP, the headspace might be off.  

Also, I highly recommend taking notes... I've learned that lesson.. "Now what did I do...?  Did I try that already...?"

-z

That's what the precision rifle guys do.  Notes on everything from temp, wind, range, loads, mirage, etc., all kept in their log book.  All in the name of smallest MOA.    Once I get this all figured out, my plan is to trick out a Rem 700P and go for the long range stuff.

Damn, look at that moon!