Bushmaster failing to feed first round

[propel]

I have factory 450 bushmaster upper. It will not chamber the 1st round reliably. I have tried several magazines with the same results. Its clean and has about 200 rounds through it. Any ideas?

[Bmt85]

What kind of problem are you having exactly? Not feeding it at all? Starts to feed, but jams?

Is the bolt carrier going back all the way back, like normal?

Is the bolt catch sticking out too far and catching the top round in the mag?

Is the round sitting slightly angled up towards the chamber?

[Jim in Houston]

The angle of the round between the lips of the mag have been a problem for others in the past. Since the second and subsequent rounds load properly, it may be that they are positioned differently from the first round due to the recoil and "jolt", when the first round is fired pushing the second round up at a slightly different angle.

If you anticipate the problem again, take an image of the loaded mag with the first round in position. Then pull the mag after you fire it and take an image of the second round in the mag. The difference, if any, may not be very noticeable.

As an alternative, before putting the loaded mag into the rifle, give it a firm tap on the bottom against the shooting bench to make sure the first round is firmly in the mag lips. Also, use the bolt release and allow the bolt to slam home. I do this routinely on my 5.56 AR.

[Hoot]

The more rounds you put in the magazine, the greater the spring pressure. Almost every 450b magazine I've encountered utilizes the same spring that the company would use with a 5.56 version. Slide the bottom off and clipp some turns off the spring. I leave just enough turns on to push the last round (lowest pressure) into position. In some magazines, that was almost half the turns in them! You can get an idea of how many to cut off by allowing the spring to protrude out the bottom with one round in there and slowly push it up until you feel a modest amount of pressure against the feed lips. Cut the turns still outside the mag off and reinstall the floor plate. Do this before tweaking the lips. You may not have to.

Hoot