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Jay
Posts: 4,629 Senior Member
My 450 BM thread
Jay
Posts: 4,629 Senior Member
A few of us have been either in the process of building a 450 Bushmaster AR or have had some interest or curiosity about it. Rather than continue to derail Sixgun’s thread, here’s my own.
I assembled a complete AR15 lower using spare parts in my man cave. With that, I needed an upper to go on it, of course.. I’ve had an interest in a big bore AR for some time. The spirit of Jeff Cooper just kept reminding me of it. Since I already use .452 components for 45 Colt and 454 Casull, the 450 BM appealed to me. When Midway had their AR Stoner brand on sale for $280 with charging handle and BCG, I bit. Right after that, they lowered it to $240. 😣
I got my upper last week and slapped in on my lower. Nothing fancy. 6 position stock, not free floated, no muzzle brake..
While I was buying stuff, I picked up 50 pieces of Starline brass, a box of 240 grain Hornady XTP Mags, a box of 250 grain Hornady FTXs, set of dies, couple pounds of Lil Gun powder and a few odds and ends. I loaded up 4 test loads to start. 2 with each bullet, from just under max book load to right at max. I took it out yesterday afternoon to try to chronograph it and test accuracy. Everything went down hill from there.
I mounted a Simmons 8 Point 3-9x scope for load testing. It’s what I had laying around.
My chronograph didn’t get one reading out of 19 rounds fired. Kept getting errors. Tried 6’ to 10’ from the muzzle. No luck. And the gun is having some feeding issues. The XTPs dont want to feed at all. They stick half way into the chamber when chambering the first round. The hollow point is hanging up. Could be solved with some magazine tweaking. But, after firing the first round, the gun is not picking up the next round in the mag. That leads me to believe it’s short stroking with the lighter bullets. The FTX bullets feed ok about 2/3 of the time. One will hang up now and then.
In Midways customer review section on this upper, there is some talk about feeding issues and a couple mentioning undersized gas port. That very well could be the case.
Recoil is stout, no doubt made worse due to the lack of a brake and the stock that’s less than ideal for this gun. It can get unpleasant to shoot after a few rounds. This thing will be getting a brake, fixed A2 style stock and a free floating handguard soon.
So, frustrated by malfunctions, having to come off the gun between every shot, stock not set up to ride a rear bag and recoil making things less than fun, I was only able to get about 2-3” at 100 yards. The final group, max charge with XTPs, started off very promising. First 2 rounds overlapping each other. Then I threw the 3rd round 3” low-left..🤪 My ultimate goal is MOA accuracy and 2300 FPS. Time will tell...
I loaded up the remainder of my brass using the FTX bullets and just under max charge of Lil Gun. Will be taking it out again this afternoon to try again with the chronograph and see if I can get a better group. Then I’ll start working on replacement parts and trying to get it feeding reliably.
I assembled a complete AR15 lower using spare parts in my man cave. With that, I needed an upper to go on it, of course.. I’ve had an interest in a big bore AR for some time. The spirit of Jeff Cooper just kept reminding me of it. Since I already use .452 components for 45 Colt and 454 Casull, the 450 BM appealed to me. When Midway had their AR Stoner brand on sale for $280 with charging handle and BCG, I bit. Right after that, they lowered it to $240. 😣
I got my upper last week and slapped in on my lower. Nothing fancy. 6 position stock, not free floated, no muzzle brake..
While I was buying stuff, I picked up 50 pieces of Starline brass, a box of 240 grain Hornady XTP Mags, a box of 250 grain Hornady FTXs, set of dies, couple pounds of Lil Gun powder and a few odds and ends. I loaded up 4 test loads to start. 2 with each bullet, from just under max book load to right at max. I took it out yesterday afternoon to try to chronograph it and test accuracy. Everything went down hill from there.
I mounted a Simmons 8 Point 3-9x scope for load testing. It’s what I had laying around.
My chronograph didn’t get one reading out of 19 rounds fired. Kept getting errors. Tried 6’ to 10’ from the muzzle. No luck. And the gun is having some feeding issues. The XTPs dont want to feed at all. They stick half way into the chamber when chambering the first round. The hollow point is hanging up. Could be solved with some magazine tweaking. But, after firing the first round, the gun is not picking up the next round in the mag. That leads me to believe it’s short stroking with the lighter bullets. The FTX bullets feed ok about 2/3 of the time. One will hang up now and then.
In Midways customer review section on this upper, there is some talk about feeding issues and a couple mentioning undersized gas port. That very well could be the case.
Recoil is stout, no doubt made worse due to the lack of a brake and the stock that’s less than ideal for this gun. It can get unpleasant to shoot after a few rounds. This thing will be getting a brake, fixed A2 style stock and a free floating handguard soon.
So, frustrated by malfunctions, having to come off the gun between every shot, stock not set up to ride a rear bag and recoil making things less than fun, I was only able to get about 2-3” at 100 yards. The final group, max charge with XTPs, started off very promising. First 2 rounds overlapping each other. Then I threw the 3rd round 3” low-left..🤪 My ultimate goal is MOA accuracy and 2300 FPS. Time will tell...
I loaded up the remainder of my brass using the FTX bullets and just under max charge of Lil Gun. Will be taking it out again this afternoon to try again with the chronograph and see if I can get a better group. Then I’ll start working on replacement parts and trying to get it feeding reliably.
Replies
Unfortunately, I had worse luck with this thing on day 2 of attempted testing.
I took it out yesterday afternoon for another attempt at getting a group and velocity. Upon firing the second round, it broke the crosshairs on my cheap scope and the bolt stuck with the carrier partially back, but the bolt still stuck in the locking lugs. I had to use the rubber end of a pair of pliers to pry the bolt open enough to get the locking lugs to fully disengage. I saw no pressure signs on the brass and this load is a slightly lower charge than my previous test loads, about 1 grain under book max. I have not found any particular issue so far. My initial thought goes along with what I mentioned in my previous post, under-gassed. But at this point, I have no definitive evidence. So I'll try scrubbing and lubing the gun well and double check my load data and measurements before proceeding. I have also noticed when shooting on the bench, most of the time the empty brass coming out is landing on the bench right beside the gun and staying there. That tends to indicate to me that it's barely getting the spent cases out of the chamber.
After getting the bolt unstuck and getting the empty case out, I fired one more round out of it standing, unsupported. Just to see what it would do. It fired and cycled fine, leaving my spent case on the ground at about 4 o'clock, 4 feet away. I put the gun away at that point, electing not to go any further until I can check everything out, just to be on the safe side.
My Bear Creek upper is very reliable, but it does fling the brass way the hell out. The bolt lugs will dent the hell out of the unfired round in the mag if I have more than 5 rounds in the mag- but that is the only complaint.
-Mikhail Kalashnikov
I should have something time tomorrow morning to look at the gun closer and do some measuring on my loads. Take it one step at a time from there. I hope to leave enlarging the gas port as the last option. That can be a job with some pucker factor..
I might look at buffer options to see if that helps first. It already acts like my buffer spring is barely adequate at sending the first round into the chamber.
I also want want to check the gas block. It sorta looked like some soot streaks in front of the gas block. But I got no black color on my finger when rubbing it. Might have just been the finish on the barrel. I’ll remove the gas block, check the gas port and reinstall, making sure the gas port aligns with the hole in the gas block and there’s no obstructions in the gas system.
I think I am catching the big bore AR bug, I “need” another upper/ caliber to load like I need another hole in my head.
Dad 5-31-13
Dad 5-31-13
-Mikhail Kalashnikov
Some say as small as .070
Seems to me like you may need to try a different buffer spring first though maybe a heavier buffer with a A2 stock? Maybe it's coming out of lock up too quickly.
Ive done some looking around and get pretty close to the same gas port size range for a 16” 450 carbine.
I didnt have time today to take it apart and look it over. When I do, I’ll measure the factory gas port size. I’ll probably end up going with a #41 drill or reamer (.096) and an increased power buffer spring when I swap to a fixed stock. I’ll hand drill it using a pin vise or hand turn the drill press to avoid breaking the bit or reamer off in the hole. Just to be on the safe side.
I have several things planned for this thing. But, to avoid spending another chunk of money all at once and to help others here narrow it down, I’ll do it one step at a time. If I do everything at once, kinda leaves us guessing what exactly solved the problem..
Seems like that could effect volume/pressure as well as the hole diameter???
Diagnose cycling issues in this order:
1. Gas
2. Spring
3. Mass
#1 isn't often remedied at home. However, SpringCo offers several spring rates. Good people there. Call them.
#3 is a refinement process. Get the gun ejecting properly with #1 & 2. Then refine ejection with #3.
No no update on the gun yet, as I’ve been busy with holidays and work. I did order a #41 reamer for the gas port, but then had a change of heart and ordered a #45. It seemed like the 41 was at the large end of the range I’ve seen, so I decided to go with a 45 first. The #45 is .0820” vs. .0960” for the #41. It’s always easier to make a hole larger. Not so easy to make it smaller.
And absolutely correct that drilling or reaming a gas port is not something typically done at home. It is a risky job if not done carefully. Breaking a drill or reamer in the hole pretty much turns the barrel into a club. Another reason to go up in size a little more gradually. And why I’m using straight fluted reamers instead of drills. The reamer will be less likely to grab and break and more likely to follow the hole straight. At least, that my thinking.. I’ll find out when the reamer gets here.
Also, gas issues aren’t always related to gas port size, of course. Misaligned or leaking gas block, obstructions in the gas system, loose gas key or broken gas key screws and misalignment between the gas key and gas tube can all cause gas related problems, such as short stroking. Those things will all be checked when I disassemble the upper before reaming the gas port. Once all of that is checked and the gas port reamed, I’ll check it to see what ejection looks like. Since I already plan to put a fixed stock on it, I’ll tune with buffer springs and buffer from there. Based on what the gun is doing now, short stroking, then not really feeding very forcefully, I’m going with the initial plan of more gas and increased power spring. Then, if it needs a heavier buffer, that’s an easy swap.
The gas block wasn’t loose, but it wasn’t tight. There are 5 set screws on the bottom of the gas block. One large one in the center that goes into a “dimple” in the bottom of the barrel, positioning the gas block. The other 4 are smaller and just there to snug everything up. I could loosen all 5 screws using nothing but 3 fingers and a little pressure on the bit driver. Still, I didn’t see evidence of a gas leak under the gas block and the gas ports seemed to be aligned properly.
The gas gas port in the barrel measures .0700.
I took a pair of needle nose pliers to the front of the feed lips on the magazine and tweaked them up just a tiny bit. On the next test, when the round got to the end of the feed lips, it jumped out of the magazine and into the chamber. Good enough..
I’ve also noticed that this magazine tends to scratch 2 parallel lines in my brass. I took some 220 grit sand paper to the feed lips and smoothed them out a bit. Problem not solved, but better. Maybe more use will help.
Also, with multiple rounds in the magazine, it doesn’t feed the top round well. Pretty much have to yank the charge handle back and let it slam home or use the bolt release button. Which is fine. But I think it will be able to use a little stronger buffer spring when I do the stock swap. More push back, more push forward. Should smooth things out. That’s the though process, anyway..
http://shopruger.com/AR-556-MSR-450-Bushmaster-5-Round-Magazine/productinfo/90679/
Problem solved. Next time I need a new mag, I’ll look for one made by “Ammunition Storage Components (ASC).”
And thanks for the heads up on the mags. The one I got was the cheap AR Stoner I ordered along with my upper. Obviously, it needs a little work. If only Magpul would make a magazine for this application... But good to know there’s another, better option out there. 👍