AR15 450 Bushmaster Home Build Feeding Issues

Talk about the AR15 style rifles chambered in 450 Bushmaster.

Moderator: MudBug

AR15 450 Bushmaster Home Build Feeding Issues

Postby gertz7342 » Fri Nov 18, 2022 1:42 pm

Hey Everyone,

I'm new here and I'm struggling with a 450 Bushmaster build I'm currently into. Hoping to get it done before Ohio's rifle season coming up in 1.5 weeks. Just a warning, this post is going to be long as I will try to detail everything I can.

Main Issue: Bolt Cycling/Feeding issues

I have a 16" barrel, 0.936" gas junction, mid-length gas system on this build. Std low profile gas block. Initially, I thought that the bolt wasn't ejecting a spent case with a loaded magazine in the gun. I tried taking video with my phone and saw the bolt cycle, just not well because the frame rate was too low. It almost seemed to me that the casing was still pressurized to the chamber as the bolt tried to extract it. I did one shot tests with no mag, and it was jamming. Extracting the case, but no ejection. I started with standard carbine buffer weight and a flatwire spring, carbine length. I swapped the spring for an unused std carbine spring, then for a used carbine spring, all with the same issues in the single shot test. I ordered an adjustable weight buffer system and also pulled an H3 buffer from a 6.5 creedmoor build my father did. I started the adjustable weight buffer at an H1 weight, roughly 3.7oz. I got ejection at 1-2 o'clock. I swapped in my father's H3 buffer, I got a nice 4 o'clock ejection. I got bolt cycling with 3 rounds out of the magazine, but no bolt lock back. I did some other tests, mainly adjusting to an H2 weight buffer to see if that would work as well, but got a 2-3 o'clock ejection, so I adjusted the adjustable weight buffer up to an H3. I swapped back in the flatwire spring, and now I can't pick up another round out of the magazine. This issue has been very frustrating, as everything I researched on 450 Bushmaster going into the build suggested standard parts would work. I don't think the gun is undergassed for these reasons:
1.) I pulled the gas block off, and carbon buildup suggests the block is centered directly over the gas port
2.) Standard gas ports on AR barrels are 0.089" - 0.096" based on my research, and a 3/32" drill bit fits the hole perfectly and measures to 0.0938", so within spec, albeit towards the top end (i.e. more gas)
3.) When increasing the buffer weights, I immediately saw better results since I got ejection, even if it was in the 1-2 o'clock position. Furthermore, heavier buffers bettered the ejection pattern results.

My adjustable weight buffer in H3 configuration (3 tungsten weights, 3 spacers, tip and roll pin w/ buffer body) weigh 5.584oz. My father's standard H3 buffer weighs 5.619oz. From what I've researched, I should be using an H1 or an H2, but my results say otherwise.

My next steps are to confirm proper cycling minus bolt lock-back with the used std carbine spring and H3 buffer, and minimum get it ready for deer season. I have an adjustable gas block that I will also play around with next, but I'm running out of time (and ammo!)

My questions:
1.) Did I diagnose the gun issues correctly? Is it overgassed? If so, what else can I do to try and fix my cycling issues?
2.) I'm running an AR Stoner mag. I've seen some posts suggesting this may be an issue. If it is, how can I check that without another magazine? If I need another mag, what should I look for? I need it yesterday lol
3.) What is everyone's experience with rifle break-in periods? My .223 Rem (my 1st build) took a long time before it ran smooth, but it runs like butter now. Unfortunately, I don't have the ammo (nor funds and time) to shoot hundreds of rounds through this gun. It's at roughly 70 right now.

Extra info I may have missed:
I'm shooting Hornady Custom 250gr FTX and Hornady Black 250gr FTX because that's what I could get around me. Barrel is 1:16 twist for heavier rounds.

Any help is appreciated. Thanks,

gertz7342
gertz7342
 
Posts: 5
Joined: Fri Nov 18, 2022 12:37 pm

Re: AR15 450 Bushmaster Home Build Feeding Issues

Postby Hoot » Fri Nov 18, 2022 5:33 pm

Welcome Aboard OM!

IMHO, a 16" barrel should be using a carbine length gas system like the original Bushmaster product uses. Not sure why your barrel manufacturer chose a mid-length gas system for a 16" barrel. To me, mid-length is more appropriate on a 20" barrel like I use. Not all manufacturers are created equal. The term "standard gas ports on an AR system" is disingenuous. Different AR calibers behave differently. The bore of a 16" 450b has a lot more gas volume than the bore of a 16" 5.56. Gas system needs will be different. Since you're stuck with where your gas port is drilled, I'd recommend opening the port diameter up a little at a time until you get ejections with the standard weight carbine buffer and spring at near 3 o'clock. I once had a 20" barrel with a rifle length gas system and it was not reliable until I opened the port up to something like .120. If you have a numerical drill bit set, I recommend taking it to the range, along with a battery powered hand drill and the tools needed to remove your handguard and loosen the gas block enough to slide it forward and rotate it to reveal the port. Then its drill, reassemble and test, repeat. Once you get it to cycle reliably, open that port one more bit step for good measure.

Hoot
In Theory, there is no difference between theory and practice. In Practice, there is.
User avatar
Hoot
 
Posts: 5083
Joined: Tue Feb 09, 2010 9:34 am
Location: Minnesota

Re: AR15 450 Bushmaster Home Build Feeding Issues

Postby Texas Sheepdawg » Fri Nov 18, 2022 7:35 pm

Mag spring too dense? Are you running your BCG wet? (Heavy dose of lube on BCG?)
I use a Teflon spray lube and run mine wet.
You can manually break in your action by just hand cycling it a few sessions.

https://youtu.be/L29stF3IyOw
-Texas Sheepdawg

http://youtube.com/c/TexasSheepdawg21
NRA Life Member
User avatar
Texas Sheepdawg
 
Posts: 4731
Joined: Fri Nov 05, 2010 10:55 am
Location: North Texas

Re: AR15 450 Bushmaster Home Build Feeding Issues

Postby gertz7342 » Fri Nov 18, 2022 8:00 pm

I'm running it almost sopping. Shooter's Choice gun grease on the BCG slide rails and oil on just about any other surface I can think of that might have contact somewhere.
gertz7342
 
Posts: 5
Joined: Fri Nov 18, 2022 12:37 pm

Re: AR15 450 Bushmaster Home Build Feeding Issues

Postby gertz7342 » Fri Nov 18, 2022 8:04 pm

Hoot, I'm confused now. Do you think the gun is under gassed, and that's why you think I should open up the gas port? If so, what is pointing you in that direction?
gertz7342
 
Posts: 5
Joined: Fri Nov 18, 2022 12:37 pm

Re: AR15 450 Bushmaster Home Build Feeding Issues

Postby Hoot » Fri Nov 18, 2022 9:25 pm

gertz7342 wrote:Hoot, I'm confused now. Do you think the gun is under gassed, and that's why you think I should open up the gas port? If so, what is pointing you in that direction?


Because its a mid-length system where a carbine length is better suited. Because it begins to extract the spent cases and doesn't have enough uumph to pop that case all the way out, eject it and travel all the way back to where it would engage the bolt hold-open or else travel forward enough to pickup the next round and chamber it and because you had better success with a lower mass (weight) buffer. Don't confuse a gas volume level problem with a timing problem although with the mid-length gas system on a 16" 450b barrel, you're probably experiencing both. I really do advise opening up the port size. I'll make you a bet. If you wind up making the problem worse as a result of over-gassing, I'll give you an adjustable gas block to throttle it back down. I firmly believe that at the end of the day, I'll still have that gas block in my parts tray. ;) I've seen this before.

Hoot
In Theory, there is no difference between theory and practice. In Practice, there is.
User avatar
Hoot
 
Posts: 5083
Joined: Tue Feb 09, 2010 9:34 am
Location: Minnesota

Re: AR15 450 Bushmaster Home Build Feeding Issues

Postby gertz7342 » Sat Nov 19, 2022 8:44 am

Because its a mid-length system where a carbine length is better suited. Because it begins to extract the spent cases and doesn't have enough uumph to pop that case all the way out, eject it and travel all the way back to where it would engage the bolt hold-open or else travel forward enough to pickup the next round and chamber it and because you had better success with a lower mass (weight) buffer. [/color]


This is where I'm interpreting my data differently. It wouldn't extract with a standard buffer weight and stovepipe at best (w/ no mag). As I increased buffer mass, I got extraction and ejection, and could move my ejection pattern back from 1 to 3 as I increased buffer mass. Doesn't that mean I have plenty of gas?


Don't confuse a gas volume level problem with a timing problem although with the mid-length gas system on a 16" 450b barrel, you're probably experiencing both.

If these two aren't dependent on each other, how do you differentiate/diagnose one over the other? Doesn't gas volume (or since I'm a chemist, I think it's more gas elution rate) time the bolt and thus the cycling of the action?

I'm sorry for all the questions, I'm just trying to get as many details as possible. I'm not opposed to opening up the gas port, I just don't want to go that route if I don't have to.

Thanks for the quick replies though
gertz7342
 
Posts: 5
Joined: Fri Nov 18, 2022 12:37 pm

Re: AR15 450 Bushmaster Home Build Feeding Issues

Postby coyote wacker » Sat Nov 19, 2022 9:27 am

I have never opened a gas block hole and had to much gas....slow and easy with a drill making the hole larger....it will take some time to get the proper hole size but that's part of the fun building your own rifle....

Remember its going to be colder during hunting season I would rather be a little over gasses than just operating .....
coyote wacker
 
Posts: 222
Joined: Thu May 21, 2015 6:45 am

Re: AR15 450 Bushmaster Home Build Feeding Issues

Postby gertz7342 » Sat Nov 19, 2022 11:06 am

Here is a link to the barrel I got. I guess it's not a good sign that it's not available anymore:

https://www.primaryarms.com/radical-fir ... arrel-16in

And thanks coyote_wrecker. It's not that I can't or don't want to in general, it's that season is about a week away. I'll get something figured out. First shot is always great, and that should be all I need. Backup plan is the shotgun. I just want to learn more about this and how to fix it.
gertz7342
 
Posts: 5
Joined: Fri Nov 18, 2022 12:37 pm


Return to AR15 Style Rifles

Who is online

Users browsing this forum: No registered users and 19 guests