Hello, I'm brand new to the .300 blackout cartridge.  I've been lurking for a little bit while I acquired parts for my build.  Well I finally got all the parts and took it to my gunsmith for assembly.  I got the upper back today and I went to hand cycle a couple of the remington UMC FMJ rounds through it, just make sure the action function.  at first none of the rounds would chamber at all.  After adding some more CLP it started to chamber the rounds it still seems to not want to chamber rounds from time to time and it seems to be random.

here are the specs on the rifle:

Rainier 10.5" barrel w/m4 cuts
Rainier Gen I upper receiver with m4 cuts
YHM BCG w/ standard Charging handle
Larue Rail

The upper sits on a command arms pistol lower.

Magazines used were magpul pmags.

here is a pic of the issue and some parts on the upper:

Failure to feed/go into battery


Picture of barrel extension into upper.


Picture of BCG


I haven't had the chance to take it to the range to do a full on function test, hopefully get out there this weekend.  If the gun fails to function properly I'm going to be really disappointed. I was looking forward to doing a .300 blk build and so far it's let me down but I'm not willing to give up on it yet.  Help me diagnose whats going on please.

UPDATE:  I took it up to the range today, lubed the crap out of the BCG.  The upper ran nearly flawless.  I had no jams, FTFs or FTEs while shooting.  I did have one failure to feed when hitting the bolt release but i think i had one too man rounds on the magazine.

I would not attempt to cycle live rounds unless the firearm is pointed downrange at a hot range.

The BCG looks very dry.

Did you use the bolt release to chamber a round from the magazine?

Did your gunsmith check headspace?

Normally I don't chamber live rounds, but I'm fortunate enough to have a cistern in my basement that I put sandbags around, two layers on one of the sides.

The gunsmith did check head space.  I made sure to specifically ask him to do so.

I used bolt release and the charging handle.  Neither seemed to affect or even cause the problem.  While using the charging handle I made sure I had it pulled back completely and then let it go without riding it forward at all.

If it were mine, knowing that the headspace is correct, I would wet the BCG down with CLP.
With different mags attempt to fire it at the range.  When I first fire my AR's they get a dripping wet BCG.
Lube it up well, and check for proper operation. Make sure the BCG is into full battery.(use your forward assist)

Do you have any different ammo?

I use dummy rounds with no primer/powder when function checking my ARs.
I have noticed with a new barrel/extension and BCG, with dummy rounds it takes the force of the spring to chamber a round.(DRY)
It may take a little operation to wear them to each other.

You may want to call Rainier or your gunsmith and ask them.

thanks for the info, i'll update saturday with a range report.

That is new for me. maybe i'll try my lancers that i have laying around.

My P-mags have been 100% reliable.

In your second picture, is it camera angle or do the feedramps and barrel extension not line up properly?

Looks to be a small lip there.

A little dremel work may be required there if it does not function properly.

I have a bunch of mag-pul stuff and it is good gear but, while p-mags work great with 5.56 rounds they are not always the best choice for variants. try the lancers they dont have the rib down the middle like p-mags do and work great for me in my 300 blackout and 458 socom . Also regular gi mags work just as good or better than anything else I have tried.

Tried cycling using both lancer mags, and GI mags, same issue.

I use  Pmags all the time ( but I have modified the inside ridges) I  took a small sharp chisel and cut out the forward ridge so the larger Dia. bullets will fit without crossing up..

Your feed ramps are screwed up!   You have  a non m4 feed ramp barrel  extension!  Have your gunsmith fix it or send the barrel back and get the M4 cut version.

It has M4 feed ramps according to Rainier

Link to barrel manufacturer.

The second picture you posted above shows M4 feed ramps.

I'd lube the crap out of it and put some rounds thru it before I worried too much.

Is your bolt carrier dragging on the feedlips?  Might be some tolerance stacking between mag position and upper and lower fit causing the mag to insert too high.

gg, instead of you trying to determine why the upper unit wont function properly, why dont you
try talking to or returning the upper to whomever the gun smith was who built it?

If they are worth theiir salt they will get that puppie working in a proper military manner, if they
blow you off, then I personally would cease doing any business with them.

Just a suggestion.

Oh an as an aside, I have had no probelms using 20/30 round Pmags or standard 223/5.56 mags in
my 300aac.

Best wishes,

Art

The cuts are in the extension - my bad!   As you see in your first pic the bullet is hitting the back of the barrel and stoppng!   They look more like rifle cuts !  True M4 cut have more slope then rifle cuts!