I'm was out at a remote comm site earlier today and tried to send this but I couldn't get the crazy smart phone they provide to do it.
if I recall correctly, someone posted over on the Modern Sporting Rifle Hunting 30RAR
sub forum, that he had good results with regular 450b mags. Again IIRC he didn't have a problem with the bolt hitting the mag lip. I didn't verify full functionality but did succeed at pulling the bolt all the way back with a regular 450b mag. It may behave differently with live fire.
Okay, time to take back what I said.
I tried a 450b mag again tonight. When I tested it the last time, I could pull the bolt back sure enough. It did rub against the top of the right lip but it did not bind. What I didn't do was hand cycle the action forward. Just tried pulling it back and when it was locked, I dropped the mag out. I know, pretty unscientific...
Under closer scrutiny tonight, I encountered a hitch when trying to ease the bolt forward, but not what you'd expect to be the problem. The larger, lower lug didn't jam against the top of the lip. It jammed against the back of the follower. I laid both mags so that the lip on the 30RAR and the two lips on the 450b were at the same height and sure enough, the back of the 450b follower, the part that is meant to catch the bolt and hold it open when empty, sits maybe a tenth of an inch higher than the follower in the 30RAR mag. Even with the BHO release lever depressed, the bolt jams against the back of the follower. If I push the mag release button in and allow the 450b mag to drop the slightest amount, then the bolt releases. It look like it could be easily correctable with a slight rounding of the back edge of the follower, but I didn't dive in yet. I do like that fact that the 450b mag still works in both of the 450b uppers I have and I don't want to compromise that. So, this weekend, when I have some free time, I'll actually have a go at it ever so carefully.
FWIW, when I had the 30RAR at the range, I noticed that some times, pulling the charging handle back on an empty mag, the bolt would slip over the top of the back edge on the 30RAR magazine follower and fly forward. In most cases, a second yank back secured it. It only occurred once when actually shooting the last round out of the mag and given my initial loads were on the anemic end, it could just as well have been from inadequate cycling gas, but I don't know for sure.
Sorry for the bum steer on using Bushmaster factory mags, which the one I was testing with happened to be. I also have a couple of AR Stoner 450b mags that are still in the unopened wrappers and will test one of those also. I strongly suspect that the OEM for them is the same OEM that Bushmaster uses since other than the different logo on them, they otherwise look identical on the outside.
Commander, you can probably forget the AR-Comp with those 130 Barnes. With the 125 flat base SST, with a max charge that I even vibrated down, it was no where near as fast as with kklatt's AA2200, so I would not hold a lot of hope for the 130s. Also, my experience with Barnes all copper bullets, particularly HPs is that they are long for their weight. According to QuickLoad, which gets this part right, Loaded to 2.26 COL, the Barnes protrudes down into the case .452 inches. The flat Base 125 SST only protrudes down into the case .293, so technically the 125 will allow more AR-Comp charge than the 130 TSX or is it the TTSX? I think you get my concern about the impact that would have upon velocity. The velocity (2500fps) of the 125 SST at max physical charge of AR-Comp was a lot less than 2840fps, so adding 5 more grains of weight and less powder probably isn't going to help. Then again, solid copper bullets have a higher bore resistance than jacketed lead since they don't obturate as easily, solid copper being harder of the two. If added bore travel resistance gets the AR-Comp cooking more, it may make up for the charge and weight difference. I look forward to your results.