Need help diagnosing an AR problem

[Bang4theBuck]

I have a friend that lives in a zone in MI where you have to shoot a straight-walled cartridge and 4 or 5 years ago he picked up a 450 Bushmaster in an AR platform to hunt with. A couple months ago he asked me to look at his rifle as he just was not confident in the accuracy of it. He has taken a fair amount of deer, but has missed as well.
The issue I am seeing with it is that is very consistently prints low and left on the first shot out of the rifle. Between 3 and 4 inches low, and 1 to 1.5" left. Shots 2 and 3 are always back up very close to point of aim and reasonably close together (touching to maybe 3/4" apart). Here is what I've done so far:
1. Swapped scopes to a know good scope.
2. Removed hand guard to check the fit and tightness of the barrel nut.
3. Changed junk 6+ lb trigger out for 3lb aftermarket.
4. Tried ammo from a different lot (same type and weight). This ammo is known to be consistent and accurate in most other rifles.
5. I verified that the issue is not a 'cold bore vs warm bore' shift by loading three rounds in two different mags and sshotthe groups back to back without any cool down time. The first shot out of group one was low/left, the first shot out of group two was low/left.

I'm looking for AR specific feedback for the most part, please. Thanks.

 

[TennJed]

Just guessing here. Have you looked at the chamber/throat with a borescope? The first round manually chambered may have less force behind it than the ones following auto chambered. I have a 6.5 that the chamber is so tight that manually trying to extract rounds is impossible, but the same rounds chamber/extract fine in my bolt gun. Maybe there is a carbon buildup that holds the manually chambered rounds farther back from the throat. Again just guessing here

 

[Bang4theBuck]

Definately worth looking into. That is one of the only difference between rounds on and all rounds after that.

 

[cohunt]

sounds like you have a loading/magazine problem-- from what you said, the first round out of every magazine is low left.....probably has to do with how it's comming out of the mag.

Are you hitting the bolt catch/release to load the first round out of the mag? Or are you pulling the charge handle back and letting it fly?

Try both and see if it changes, also try 1 round only- 5 times in a row loading every time the same way--- see if they all print low left.

You can try tweaking the feed lips to present more bullet up too-- sometimes that first round out of a loaded mag really hits the feed ramps hard if the feed lips are too tight

 

[stinkyduck13]

so then maybe you could try three in a row that are by hand?

 

[Bang4theBuck]

All great suggestions. I'm boresciping now and will look at feeding next

 

[ShtrRdy]

After trying a repeated "first round from mag" test, if the problem is still there I would look at the connection of the barrel to the receiver and the forend to the receiver. The heat from a first shot could be changing where the barrel is pointing.

I have to ask this - is the scope mounted directly to the top of the receiver? A picture of the rifle might help us.

 

[coyote wacker]

I handload have never fired a factory round in the 450 Bushmaster.....loading a round by dropping the bolt exerts a lot of pressure almost like using a kinetic bullet puller....another thing to remember theres no crimp grove that can be used to hold the bullet in place also the case head spaces on the cartridges lips...dropping the bolt on a loaded 450 Bushmaster is not like doing it to a 5.56 with a knurled bullet and crimped in the groove its very hard to move....

What I have found is don't charge a round by just dropping the bolt....I ease the bolt down using the charge handle then use the forward assist to seat the bolt down on the cartridge....

One thing to remember when hunting everyday the gun is loaded and unloaded after several times the OAL of a cartridge will get longer until after several times the bullet will get jammed into the rifling and raising pressure and changing accuracy....

 

[cajun]

Rack a round from mag. Extract it and measure it see if it's oal is changing.

 

[waveslayer]

Rack a round from mag. Extract it and measure it see if it's oal is changing.

if that's the case with just the bolt release all the rounds should have a OAL change that are sitting in the mag from the recoil... The bolt closing doesn't have enough force to push a bullet out, if it does, he's not crimping enough. Like the OP said, if he is using factory ammo then it's not the Crimping.

How many people have shot this rifle and have the same results?

 

[waveslayer]

I have a friend that lives in a zone in MI where you have to shoot a straight-walled cartridge and 4 or 5 years ago he picked up a 450 Bushmaster in an AR platform to hunt with. A couple months ago he asked me to look at his rifle as he just was not confident in the accuracy of it. He has taken a fair amount of deer, but has missed as well.
The issue I am seeing with it is that is very consistently prints low and left on the first shot out of the rifle. Between 3 and 4 inches low, and 1 to 1.5" left. Shots 2 and 3 are always back up very close to point of aim and reasonably close together (touching to maybe 3/4" apart). Here is what I've done so far:
1. Swapped scopes to a know good scope.
2. Removed hand guard to check the fit and tightness of the barrel nut.
3. Changed junk 6+ lb trigger out for 3lb aftermarket.
4. Tried ammo from a different lot (same type and weight). This ammo is known to be consistent and accurate in most other rifles.
5. I verified that the issue is not a 'cold bore vs warm bore' shift by loading three rounds in two different mags and sshotthe groups back to back without any cool down time. The first shot out of group one was low/left, the first shot out of group two was low/left.

I'm looking for AR specific feedback for the most part, please. Thanks.

I would go visit an AR specific Forum to get the best help. Not saying guys on here don't know, but there that's all they do

 

[coyote wacker]

I would go visit an AR specific Forum to get the best help. Not saying guys on here don't know, but there that's all they do

Want to get a answer from people that have use the AR-15 450 Bushmaster long before it was allowed for hunting in many States .......
450 Bushmaster.net

Have done more work developing guns and ammo for the 450 Bushmaster in Michigan than anybody.....my first gun in 2009 years before they were allowed to the used in the Southern Deer Zone I used one in the UP .....having worked with bullet suppliers to develop bullets for the 450 Bushmaster only....

I 'am going to guess the upper the OP's friends gun was built by or parts from Bear Creek Arsenal they have many problems and some are just plain junk....but are very good at repair or replacement of there troubled parts and very inexpensive to buy....sort of a roll of the dice buying a gun or parts from them....

 

[waveslayer]

Want to get a answer from people that have use the AR-15 450 Bushmaster long before it was allowed for hunting in many States .......
450 Bushmaster.net

Have done more work developing guns and ammo for the 450 Bushmaster in Michigan than anybody.....my first gun in 2009 years before they were allowed to the used in the Southern Deer Zone I used one in the UP .....having worked with bullet suppliers to develop bullets for the 450 Bushmaster only....

I 'am going to guess the upper the OP's friends gun was built by or parts from Bear Creek Arsenal they have many problems and some are just plain junk....but are very good at repair or replacement of there troubled parts and very inexpensive to buy....sort of a roll of the dice buying a gun or parts from them....

Good advice on the Bear Creek Arsenal...I was going to add if the barrel was bedded or the upper was trued to the barrel etc... So many more factors when it comes to a gasser.

I wasn't questioning your knowledge on AR's or experience... I shot a bunch of of his 450 Bushmaster, I felt bad blasting away when the ammo cost was crazy compared to a standard AR.

Most likely the OP's friend needs better components and someone to put it together for him. Barrel could be a big factor as well for the poor performance, most likely the biggest culprit, then how it's mounted or trued to the upper etc..

Thank you for your service with developing bullets.