Recently built a 300 BO upper. I loaded 3-4 shells at a time in an old 20 round mag. It fired two and jammed on the 3rd one several times. I was going to bring several mags to try. Just wondering if it is the magazine or my weapon. I think it is the magazine. What do you guys think. Got to go back. Oh, the loads were 125 gr. Nosler ballistic tips loaded to a grain below max. I only fired about 6 rounds, two three times. I went to the range at the last minute with a friend and grabbed my stuff. I have got to go back during the week when it is not so crowded.

what type of jam?

First thing you always need to do is eliminate the magazine as your suspected point of failure.

After you've do that, give us a better idea of what's happening and we will see what we can do to help.

Also, since these are handloads, what overall length are they loaded to?

What length of gas system?

It is carbine length. Reloads were just short of maximum length. I've never had trouble with any of my reloads of other calibers, always go bang and usually print a dime at 100, at least my 308 does. I think it is that one old magazine. I'm taking others next time. Oh, it would shoot two, and jam the third about half way in. I was only loading 3 at a time to get an idea of the group. May need to load the whole mag and use another mag. I also am going to take this mag apart and stretch the spring to give some tension back. I've also built two others in 223 and haven't had any problems, but I found a few old 20 rounder mags, and thought I would use them on the 300 to be different from the 223 mags, for a little more safety.

Steel mag?

There were a couple of bad contracts, who's name in on the bottom?

I've been busy. Bottom of magazine says:
SANCHEZ ENT.
MANSFIELD, OH
USA

It also has a circle with DSL inside of it. It also has a black follower.

get a new magazine

Cooper and Sanchez are ones to stay away from. Some may work fine but they had enough problems to be noted and Cooper lost a contract.

I've always used the cheap polymer 20 round Canadian Military mags in both of my .300 Blk out AR-15's. Never had a malfunction but I've only shot factory ammo in my guns. I'd try a different mag, first.

Pmags are $11, you'd be crazy to try and stretch a spring to get a dud magazine to work.

There is a proscribed fix for the Cooper and Sanchez magazines.

1. Lay magazine on pavement.
2. Drive over magazine with Suburban, D-9, Humvee, Striker, Bradley, Abrams, or other appropriate vehicle.

my BO is fine with Pmags but I prefer Arid Extra Dry for the problem.

Don't know when I will get back to the range. I have to complete my honey do list first, probably next week. I will take some various mags including Magpul mags to see if that is it. I used 125 gr bullets and installed them a little shy of max length since I was almost max on powder. No marks on bullets when loading. I also may load 10-15 instead of 3 at a time to see if more tension helps push them up. Sweet shooter, no kick, and just a bang not a loud crack.