450 BM Double-Cycling?

Talk about the AR15 style rifles chambered in 450 Bushmaster.

Moderator: MudBug

450 BM Double-Cycling?

Postby papajohn » Mon Jun 25, 2018 1:55 pm

I have a 450 Bushmaster I got several years ago, and then upgraded it a bit later to add a muzzle brake, something it badly needed, because shooting this AR-style rifle off a bench is not real pleasant. It took awhile to get everything lined up properly, but yesterday was the first time out with the muzzle brake, and things went well.......mostly.

Image

The loads were 250 grain gummy-tips and 300 grain softpoints, the heavy bullets didn't shoot as well as I would have liked, so when they were gone I got out the loads with the 250 grain Hornady FTX's.

Image

I wasn't going to dial the scope in until I had settled on a load, so when groups were left-of-center I didn't worry about it. I settled in behind the scope and touched off three rounds, all careful-like, and was rewarded with a nice oblong hole of about .70". I slid back behind the scope and pulled the rifle in tight, started taking up the pressure on the trigger, then a little more, then..........BLAM BLAM!

Yes, the rifle doubled. Happily, there were only two rounds left, the first shot went right into the previous hole and the second was a half-inch left of the rest, and I was still trying to figure out whether this was worth having a heart attack over. I was pretty freaked out. I've shot machine guns before, several actually, but this was totally out of the blue, and thankfully my follow-through was good enough to keep the gun pointed at the target.

Image

All this was at 50 yards, mind you, I don't generally shoot at 100 until I'm fine-tuning the loads.

After having a day to think about it, I don't thing it was the gun's fault, it was more mine. I think I acted like a bump-stock, the gun recoiled into my shoulder while my trigger finger stayed where it was. So the trigger reset, and as the rifle bounced back off my shoulder it met my still-stationary trigger finger, and pushed into it hard enough to fire. It happened so fast that I couldn't react, but I do remember looking to my right and seeing two large gleaming hunks of brass, both still airborne, sailing over the bench next to me to clatter on the ground 20 feet away. It was pretty surreal.

Has anyone here experienced this kind of thing before? I've been shooting AR's for about 25 years, but this is the first one with enough recoil to cause this issue, I'm convinced a softer-shooting gun wouldn't be apt to double this way.

I can assure you that if it had happened with a full magazine, things would have been pretty skeery. When the 450 BM was brand new several years ago, a guy from my range bought one and put it in a bump-stock. I was there the first time he shot it with full-power loads, he was trying to keep his shots in the dirt berm at 25 yards and none of the bullet impacts were within 10 feet of each other. The shooter was tall and thin, and while he was hip-shooting it he looked like someone had hooked him up to a 220 volt power line, he was getting shaken and stirred pretty badly until the mag finally ran dry. :o
papajohn
 
Posts: 32
Joined: Tue Jan 12, 2016 3:10 pm

Re: 450 BM Double-Cycling?

Postby MOOSE EARS » Mon Jun 25, 2018 3:35 pm

What trigger are you using? I had the same thing happen with a new drop-in trigger in a 375SOCOM that I wasn't familiar with. I also had it happen with a modified milspec trigger that I set up too lightly. Heavy recoil adds to the potential, obviously.
MOOSE EARS
 
Posts: 78
Joined: Sun Nov 17, 2013 3:50 pm

Re: 450 BM Double-Cycling?

Postby Bmt85 » Mon Jun 25, 2018 5:34 pm

I believe it could happen, the 450B has some pretty good recoil sometimes. I personally haven't experienced this, and that's running Geissele SD-C and SD-E triggers with my 450B's, one of which has no muzzle device. I also shoot some pretty stout loads, too.
Bmt85
 
Posts: 372
Joined: Wed May 18, 2016 11:13 am
Location: S.E. Michigan

Re: 450 BM Double-Cycling?

Postby papajohn » Mon Jun 25, 2018 5:43 pm

The trigger is a Timney I installed into that lower back in 2011, it's factory-set for four pounds and can't be adjusted.
papajohn
 
Posts: 32
Joined: Tue Jan 12, 2016 3:10 pm

Re: 450 BM Double-Cycling?

Postby MOOSE EARS » Mon Jun 25, 2018 5:59 pm

Bmt85 wrote:I believe it could happen, the 450B has some pretty good recoil sometimes. I personally haven't experienced this, and that's running Geissele SD-C and SD-E triggers with my 450B's, one of which has no muzzle device. I also shoot some pretty stout loads, too.


2 stage triggers are not as likely to have this happen. Much more likely with reduced weight single stage drop-ins.
MOOSE EARS
 
Posts: 78
Joined: Sun Nov 17, 2013 3:50 pm

Re: 450 BM Double-Cycling?

Postby Hoot » Mon Jun 25, 2018 8:36 pm

Personally, I'm impressed that the second shot in the double wasn't high! If you were holding it that tight, I'm amazed that it "bumped" on you. Never had a double as both my 450b lowers use RRA NM 2-stage triggers as does my 30 Rem AR lower. I still have a couple of my GI single stage trigger parts, somewhere in the morass of my shop in a drawer. I hoard, therefore I am. ;)

Hoot
In Theory, there is no difference between theory and practice. In Practice, there is.
User avatar
Hoot
 
Posts: 4054
Joined: Tue Feb 09, 2010 9:34 am
Location: Minnesota

Re: 450 BM Double-Cycling?

Postby plant_one » Mon Jun 25, 2018 10:13 pm

papajohn wrote:The trigger is a Timney I installed into that lower back in 2011, it's factory-set for four pounds and can't be adjusted.


are you using a standard rifle buffer and buffer spring in your lower?


if you're super over gassed - a timney can exhibit that symptom . from what i recall timney telling me its a bolt carrier velocity issue, combined with bolt bounce.
i suspect the brake probably put a little more unf on things and pushed you over the edge.

just to be clear - this is not a 450 bushmaster issue, its an improperly tuned gas system issue - and a carbine length gas system on a 20" barrel is a recipe to be over gassed. i experienced it with a 20" rifle length gas system 223 when i first tuned up my suppressor on it.



u should be able to prevent it in the future if you increase your reciprocating mass by a notch - if you're already on a standard rifle buffer, probably a CCWS from hoot will be the recipie :)


my 450 bushmaster can easily cycle a h2 buffer with a ~4oz CCWS slug in the bcg too - or 8.6oz + BCG . Slash's (Heavybuffers.com) makes a 11oz buffer for big bores to manage the gas system if you want to go that route and make the $125 investment for it.
User avatar
plant_one
 
Posts: 623
Joined: Sat Mar 19, 2016 8:58 pm
Location: Oakland County, MI

Re: 450 BM Double-Cycling?

Postby papajohn » Tue Jun 26, 2018 12:08 pm

I stated earlier that I'm convinced the rifle doubled because of the way I was holding it, my body acted as a bump stock. Yet you're convinced this is because of a gas system issue? I'm running a standard buffer and spring, which, as I understand it, is what the gun was designed to be run with, using standard ammo.

I've done some (very little) reading on the CCWS and I understand what it's for and how it's supposed to work, but I'm not convinced that's the cure here. It would seem that tweaking the loads would be the simplest approach, unless I'm missing something. The fast that I've fired several hundred rounds and it's happened exactly once tells me that it's not a major problem.

It would seem that more testing is in order with the same loads and a bit of experimenting with bench positions would be the next order of business.
papajohn
 
Posts: 32
Joined: Tue Jan 12, 2016 3:10 pm

Re: 450 BM Double-Cycling?

Postby plant_one » Tue Jun 26, 2018 7:35 pm

papajohn wrote:I stated earlier that I'm convinced the rifle doubled because of the way I was holding it, my body acted as a bump stock. Yet you're convinced this is because of a gas system issue? I'm running a standard buffer and spring, which, as I understand it, is what the gun was designed to be run with, using standard ammo.

I've done some (very little) reading on the CCWS and I understand what it's for and how it's supposed to work, but I'm not convinced that's the cure here. It would seem that tweaking the loads would be the simplest approach, unless I'm missing something. The fast that I've fired several hundred rounds and it's happened exactly once tells me that it's not a major problem.

It would seem that more testing is in order with the same loads and a bit of experimenting with bench positions would be the next order of business.



call timney - that was their answer to me when i approached them about the issue, i surely didnt come up with it on my own :)
User avatar
plant_one
 
Posts: 623
Joined: Sat Mar 19, 2016 8:58 pm
Location: Oakland County, MI

Re: 450 BM Double-Cycling?

Postby Hoot » Tue Jun 26, 2018 9:20 pm

This is not the end of civilization as we know it.
You shoot autoloaders, You'll get an occasional bump. My Browning BAR in 300 WM does it to me on occasion. Autoloaders jump around. If your trigger technique is as it should be, slow, steady and deliberate, they may reset before you go past the trip point. Its not the gun's fault, the ammo's fault, the fill in the ______________'s fault. it just happens. 'nuff said...

I'm still marveling at the excellent group! Image

Hoot
In Theory, there is no difference between theory and practice. In Practice, there is.
User avatar
Hoot
 
Posts: 4054
Joined: Tue Feb 09, 2010 9:34 am
Location: Minnesota


Return to AR15 Style Rifles

Who is online

Users browsing this forum: No registered users and 8 guests