Failure to feed problems - PMAGs, 300/221

[JFettig]

I've been shooting my 300/221 AR pistol quite a bit lately and have been having a lot of failures to feed, even with PMAGs, actually especially with PMAGs. I'm pretty sure I know what it is. I think its because the goofy bullets I'm shooting have a super long ogive and they aren't contacting the mag to pop them up on the way out so the tips are staying low and hitting below the barrel extension feed ramps. I've also had a couple with 125gr TNT bullets.

Would the proper thing to fix this be to cut M4 style feed ramps into the upper/extension?

I haven't tried enough different mags to know if it only happens with PMAGs, its just that I have used them 95% of the time and haven't had it happen the other 5% of the time. I've been using the 20rnd PMAGs, I have 4 20s. It typically happens to the first couple off the top of the mag, I'd say within the first 5.

The gun is cycling just fine, enough power for the buffer to slam against the back of the tube(not too much though).

I don't know the proper way to cut M4 feed ramps but I'm pretty good with rotary tools and machining equipment.

Any pointers/help would be appreciated.

Jon

[cav_scout_tj]

Since Pmags are so easy to take apart, do that. Clean up the flashing on the followers with some a file or knife or sandpaper. make sure they flowers are like glass from bottom to top. My Pmag 20s were much rougher than 30s, there have been a few threads on that. Some powdered lock smith graphite lubricant is also a good idea for all mags.

Linky

Also, what bullets are you using and whats you COL?

[JFettig]

I just recently read that thread but never got around to it, I'll give it a try.

My OAL is 2.245"

Jon

[kurtz]

Hi Jon,

I had my first 'fail to feed' (3 rounds out of 60) with the 20 round PMAGS and 125 TNTs today, sounds like a similar problem as you, the hollow tip was hitting right at the transition point from the receiver to the extension ramp....was testing an upper that does not have M4 type extended feed ramps....the 30 rd PMAGS ran 100%....I have noticed that the 20 round PMAG followers are able to tilt much more than the 30 rounders, not sure if this is compounding the problem just an observation.....I put the 30 round followers in the 20 round body, worked great although the mag lost a couple rounds of capacity due to the longer follower....

I used a dremel tool with a 3/16 tapered polishing point, the one that looks like grey rubber, not the stone type, to smooth out the ramp and transition area......seems to have helped so far, heading to the range again tomorrow to try the mag follower exchange and polished ramps...

kurtz

[JFettig]

I just pulled one mag appart and taking that flashing off did help with the follower hesitating, I noticed that it looks like the tips are dragging on the front of the mag.

I did have one round actually jam up into the front of the mag and not into the extension/receiver.


Jon

[Ron458]

With a round in the mag & looking from the front. The bullet point needs to be about a 1/16" above the the front lip. If it is then insert in lower & with the upper on ease bolt forward slowly. Were ever the point of the bullet contacts must be modified. It must have somewere to go at all times. It sounds like you are on the right track. M4 cuts will help with the transition. If it gets stuck in the mag then the follower is not pushing it up fast enough during firing or the point is not high enough. I use the NHMTG 20s & have never had any problems. Some mags just sit lower in the magwell & are problematic. Another idea is the 6 O'clock lug on the extension can be smothed out. By knocking this down some it softens the jump from the mag.
Have you tried any other mags?
Sorry this is basic info but sometimes you have to start at the begining.

[Pitt300]

If loading 208,220 or 240s to 2.260, you need to use polymer/plastic mags like MagPul or others.
Take them apart & w/a file remove the front ridge on both sides of the mag that would normally press the bullet of a .223 into the center of the mag so that it will feed out of the center when it gets to the top.
Mark the mag for LONG/HEAVY loads. These will now NOT work as well w/125g loads BUT they will feed your heavy loads great. You may also have to lower the feed lip on the mag a few thousandths w/a round file to help guide them out into the chamber/feed ramp.

Otherwise, seat to a COAL of 2.1??? or shorter & use the mags as they are.

[JFettig]

I've only tried bushmaster 20 rnd mags, I have not put many rounds through it.
Here is a picture of a round stuck down which I've had once or twice.

Pitt, is the circled rib the one your talking about?

Jon