Not to get bogged down into a useless done a skillion times before off topic debate, a correctly assembled, correctly lubricated, in-spec rifle using decent ammo and good mags will work for beginners and advanced shooters alike. :banghead: Right?
My 115#, 20YO daughter works her's fine, there are 15 gazillion of them in the hands of ill-to-moderately trained grunts around the world, they're used by our own elite forces at many levels - must be at least passable reliable. They really don't even have to be especially clean to run well.
I cannot say they are 100%, first time, every time - in my experience, a brand new rifle will be a little sticky and the bolt/bolt carrier should be run wet with lube for the first hundred rounds for best function, but after that it will be GTG with normal, sparingly applied lube. Know this, accept it, make your expectations real and move on.
IMHO, anyone that disagrees is probably citing old ('68) info or is interjecting an irrelevant 'torture test' scenario into a limited-scope, range function discussion.
In any case, the doggone mags should insert, even if the rifle did jam every other round, like we all heard about 40 or 50 some-odd years ago.
This is just my opinion, but I have two Colts, three Bushmasters and three Fulton Armory rifles, some built myself, others factory assembled and all of them apart and together a half-dozen or more times to base this opinion on.
If I ever do have a problem, it always comes back to the ammo or the mag. My daughter's Colt and one of the Bushies have not been cleaned since BulletFest in June, and they both have several thousand rounds through them since then with no hiccups, both Q3131 & the dreaded Wolf. I also have an AK, several SKS's and an M1A, so I'm really not locked into any particular platform and thus have no dog in the fight.
That said, we can go back on topic, and the problem should be simple enough to diagnose.
Sorry if I repeat info from above posts, but one should go through all of the steps and I didn't want to miss one.
First, using more than one USGI magazine should essentially rule out a magazine problem, unless they've been mashed together or both are from a batch of bad ones. Assuming they're clean and dry inside, of course. If any doubt, try those mags in another rifle, or try a few more USGI mags in your own rifle.
If your 30 round mags are older and do not have "green followers", there is an off chance that the mag springs are binding, and this will make it very tough to insert under a closed bolt. Go to green followers and/or use another mag with a green follower to recheck the rifle. Your USGI 20's should be fine with the regular alloy followers, or the later black plastic.
Otherwise, it's the lower receiver. To check the lower, remove the upper and try inserting an empty magazine.
The empty mag should slide freely into the mag well w/o binding and the mag catch should ride over the feed lip and move out of the way smoothly and freely, smartly snapping into place when the mag gets into position. When the mag release button is depressed, the mag should fall out. Anything else is wrong, right?
If the mag binds, see where it is binding. If it gradually starts getting tighter as it bottoms out, the magwell may be distorted. Do the mags show marks from the bind? Look for scratches where a burred mag catch might not want to ride up over the left side feed lip. Look for bright spots on the corners where the mags or magwell might be out of square. The magwell itself might be out of square or it is not to specified size, both factory defects. It can happen, both Colts I have are square, but a little tight in the magwell and some USGI mags do not "drop free", while they fall cleanly from the Bushies.
Here is a drawing of the AR15 lower if you want to look at dimensions:
AR-15 CAD- Lwr Rcvr-PDF
If the mag suddenly gets tight at the end of it's insertion, remove the mag catch and try again. If the bind went away when the mag catch was removed, the mag catch is the problem. Check that the mag catch is clean and smooth on the edges and there is sufficient clearance when you press the button. It should operate smoothly with finger pressure on the mag release button. With the button & spring removed, the catch itself should fall in and out of the opening-almost 'sloppy' in fit.
If there is any binding of the mag release, modify the release only, do not touch the receiver with tools, unless you'd like to own this problem yourself. Look for burrs and rub marks. Relieve any areas that contact the receiver opening and hit them with touch-up blue. The mag catch adjustment should be made in such a way that finger pressure on the button moves the catch completely out of the way, once that's done, there really is no more adjustment - it either works or not.
If there is no binding with the lower receiver separated and function checked with an empty magazine, but binding occurs when the rifle is assembled with a loaded mag, it is possible that the mag catch is positioned too high. That would be a factory defect in the lower. Consulting the CAD drawing linked above and measuring will answer that question.
It is mentioned above that one should FIRMLY bottom out the mag and I would expect considerably more resistance to inserting a loaded mag under a closed bolt (tactical reload), but it definitely should go, and the rifle should continue to run just fine - the added resistance should not have enough effect to slow or stop the bolt carrier's progress.
It is unlikely that the upper receiver or any of it's components are causing the problem - if the rifle will fire and cycle with a half-magazine full of ammo, the upper has to be correct, or it would bind at the hammer or buffer tube and malf at any time.
Bushmaster is very responsive to service complaints and in all cases I'm aware, they have repaired the rifle and shipped it back very promptly. The only part I'd suggest modifying is the mag catch itself (if necessary), an easily replaced $3 part. If there is any problem with the magwell or the mag catch position, reassemble the rifle and send the whole deal back to BM, let them make it right for you.
Hope this helps.
