Bullet velocity anomaly

[mkrem]

I witnessed something odd yesterday while shooting a 223 with a BA 20” premium series barrel 1/8 twist. While doing some group work and measuring velocity with a labradar, I witnessed my bullet velocity speed up (2840-2940 fps) with each firing over the 6 shots fired. The 6th shot was throw about 5” wide of the original POI. Shooting 77 gr smk

A few years ago, I witnessed something similar to this with this same barrel. I was shooting at the center of a lake looking for splash and the bullet impacts just disappeared after a certain point. Shooting 55 gr bullets.

Has anyone experienced this significant of POI shift? I think the barrel being hot was the reasoning behind the lake experience. But, still had 50 yards of lake in either direction of my POA so this bullet was going wildly off course. But, yesterday shooting 6 shots over the course of 2 minutes, shouldn’t be enough to affect a barrel like this. This isn’t a pencil barrel, it is a heavy barrel. Both the loads, 55 gr and 77 gr bullets, produced sub moa groups originally before going crazy.

 

[bfoosh006]

Were those reloads ? or Factory ammo ?

 

[Oryx]

100 fps delta is easy with a sloppy hand load

 

[hlee]

“Heat soak” and temp sensitive powder?

 

[Oryx]

Some stick powders that crunch through a powder drop without weighing every load can produce that. Mixed brass with different internal capacities. Large swings in neck tensions. temp sensitivity of powder. a mix of 100 uncontrolled variables

 

[mkrem]

100 fps delta is easy with a sloppy hand load

All hand loads. These are precision loads, not “sloppy”. In another rifle, they provide an sd of 7 over the course of 10 shots. All charges are run on a chargemaster. Lapua cases, cci 400, 77 smk, tac 24.8 gr.

The load is not my concern. As I noted, this is happening with another load as well in this rifle. Other rifles do not exhibit this issue.

“Heat soak” and temp sensitive powder?

heat soak should not be an issue with the rate of fire undertaken and the amount of rounds fired. But it crossed my mind also. 6 rounds in 2 minutes.

My thought is barrel defect. But what kind of defect I have no clue. I’m not well versed in that area.

 

[hlee]

 

[mkrem]

View attachment 7821000

What is your preferred barrel manufacturer?

 

[hlee]

What is your preferred barrel manufacturer?

My JP barrel is a laser, but I've been working with a barrel from AR15performance recently that is really good too. For an off-the-shelf solution that just works? JP every time. I've heard mixed reviews of Odin Works, but I've got a couple that I have been very happy with. If I were building a no-compromise rifle, I'd go with a Bartlein blank chambered by Craddock Precision.

 

[mkrem]

My JP barrel is a laser, but I've been working with a barrel from AR15performance recently that is really good too. For an off-the-shelf solution that just works? JP every time. I've heard mixed reviews of Odin Works, but I've got a couple that I have been very happy with. If I were building a no-compromise rifle, I'd go with a Bartlein blank chambered by Craddock Precision.

I’ll have to try those barrels out. I primarily have BA and faxon. A chambered blank isn’t worth the cost to me in an ar platform.

 

[Earnhardt]

What powder?
Grains?
Compression ratio?
COAL?

 

[mkrem]

What powder?
Grains?
Compression ratio?
COAL?

see above. Not sure on compression, never heard of a way to measure that. No powder is crunching. Cbto 1.863. Coal is mag length

 

[FALex]

What is your preferred barrel manufacturer?

I have seen many people give BA barrels praise. I have only seen and shot one AR that was chambered in .308 by BA. The damn thing could not shoot many different types of ammo (including match rounds from Federal and Hornady) under 1.5MOA. To be honest, 1.5 isn't necessarily poor performance, but with the amount of praise I have observed, I expected much better.

I know you weren't asking me, but opinions are like assholes, so here goes:
Bartlein, Krieger, JP, White Oak

As @hlee stated, a Bartlein chambered by Craddock is probably going to be the pinnacle of AR precision, and I say that as a huge, dyed-in-the-wool JP fanboi. The only reason I say that, is because @bigjake83 and I shot his .308 that had been chambered by Paul at Craddock, and that thing put together some, "holy shit, that rifle shouldn't be able to do that" type of groups.

As far as your problem goes, are you feeling consistency in the press when seating your bullets? What type of seating die are you running?

 

[mkrem]

I have seen many people give BA barrels praise. I have only seen and shot one AR that was chambered in .308 by BA. The damn thing could not shoot many different types of ammo (including match rounds from Federal and Hornady) under 1.5MOA. To be honest, 1.5 isn't necessarily poor performance, but with the amount of praise I have observed, I expected much better.

I know you weren't asking me, but opinions are like assholes, so here goes:
Bartlein, Krieger, JP, White Oak

As @hlee stated, a Bartlein chambered by Craddock is probably going to be the pinnacle of AR precision, and I say that as a huge, dyed-in-the-wool JP fanboi. The only reason I say that, is because @bigjake83 and I shot his .308 that had been chambered by Paul at Craddock, and that thing put together some, "holy shit, that rifle shouldn't be able to do that" type of groups.

As far as your problem goes, are you feeling consistency in the press when seating your bullets? What type of seating die are you running?

I appreciate your response and opinion on barrel brands. I’m just not ready to fork over $550+ for an ar barrel right now. I’ll probably send this one in and see if they can find something wrong.

Regarding the press, neck tension is consistent. I’m using Redding type s bushing dies.

 

[Oryx]

What was the chrono data for 6 rounds ... Hard to blame a barrel for walking velocities over 6 shots. I'd definitely look at the ammo for the swings in velocity.

I'd look somewhere else for the shot that was 5" wide of the group - Especially if you're claiming the barrel already produces sub MOA results. My first guess after 6 shots would be the shooter - we all screw up occasionally.

Either way, 6 shots isn't really significant enough to consider anything a problem.

 

[mkrem]

In order of shot 1-6, velocity was 2843, 2866, 2900, 2912, 2926, 2940.

I agree that we all screw up and pull shots. But I am able to recognize when I do so. I shoot precision rifle and am accustomed to 3/8 moa and less groups when I’m shooting off the bench. Me pulling a shot will make that 1 moa. So, what I’m saying is I didn’t pull the shot 5”.

 

[FALex]

I hate to ask the obvious, but you've checked that all parts have not come loose? Scope rings, bases, any muzzle devices, barrel, etc...?

I agree with you, pulling a shot is one thing, but 5 inches is quite a bit of pulling.

 

[r.tenorio671]

In order of shot 1-6, velocity was 2843, 2866, 2900, 2912, 2926, 2940.

I agree that we all screw up and pull shots. But I am able to recognize when I do so. I shoot precision rifle and am accustomed to 3/8 moa and less groups when I’m shooting off the bench. Me pulling a shot will make that 1 moa. So, what I’m saying is I didn’t pull the shot 5”.

.....those are some pretty large swings in velocity, I'd want to tighten that up before I'd declare the barrel as the sole source of the 5" deviation for that last shot. Curious, if you didn't count that outlier #6, what was the group looking like?

 

[mkrem]

I hate to ask the obvious, but you've checked that all parts have not come loose? Scope rings, bases, any muzzle devices, barrel, etc...?

I agree with you, pulling a shot is one thing, but 5 inches is quite a bit of pulling.

Yes sir, after # 6 it went back to center.

.....those are some pretty large swings in velocity, I'd want to tighten that up before I'd declare the barrel as the sole source of the 5" deviation for that last shot. Curious, if you didn't count that outlier #6, what was the group looking like?

The group was solid, close if not under 1 moa. I don’t recall exact size atm.

I’m going to shoot it again today under the labradar and see what it does.

 

[357Max]

I would add Proof Steel to the above recommended list. They are consistently accurate AR barrels
In the $350-$450 range.

The only thing I can think of that should be ruled out is bullet set back.

With each round going higher it would make sense. 6th round x6 recoils etc.

How much interference fit are you running on these loads? Try adding .001 smaller bushing or mandrel & see what happens.

Edit: @mkrem - If I understood correctly, you fired 6 from a mag with each speeding up.
Then it went back to center with first shot new mag. Sounds like set back to me.

 

[mkrem]

I would add Proof Steel to the above recommended list. They are consistently accurate AR barrels
In the $350-$450 range.

The only thing I can think of that should be ruled out is bullet set back.

With each round going higher it would make sense. 6th round x6 recoils etc.

How much interference fit are you running on these loads? Try adding .001 smaller bushing or mandrel & see what happens.

Edit: @mkrem - If I understood correctly, you fired 6 from a mag with each speeding up.
Then it went back to center with first shot new mag. Sounds like set back to me.

Yes, you understood that correctly. The 7th shot went back to center with a velocity of 2893. Can you explain what you mean by setback?

I’m assuming by interference fit you are referring to what I call neck tension. If so, I’m running about .004 since it’s in an AR. I typically run .002 in my bolt guns.

 

[Oryx]

2893fps would still be 50 fps higher than your original starting velocity or right in the middle of your spread. The first round out of your fresh mag wouldn't necessarily have setback from recoil

Set back would be be the bullet going further into the case resulting in potential higher pressures (and potentially velocities). If there isn't enough neck tension the bullet can potentially set back in the case under recoil.

.04 interference fit should be sufficient to hold the bullet in place, but it's really just a mathematical calculation and doesn't take into account brass condition/spring-back. If brass is too soft it may not hold that tension.

Setback can also happen from feeding out of the mag if barrel ramps are not cut correctly, don't correctly align with receiver ramps, or just poorly feed out of the mag.

 

[357Max]

Yes, you understood that correctly. The 7th shot went back to center with a velocity of 2893. Can you explain what you mean by setback?

I’m assuming by interference fit you are referring to what I call neck tension. If so, I’m running about .004 since it’s in an AR. I typically run .002 in my bolt guns.

@Oryx nailed it. That is what I meant by set back. I’ve started using the term interference because that is really the correct term to use.
Just having .004 press fit does not in reality equate to tension for the reasons Oryx described. Brass spring back etc.

Is it a DPMS lower by chance?

 

[mkrem]

@Oryx nailed it. That is what I meant by set back. I’ve started using the term interference because that is really the correct term to use.
Just having .004 press fit does not in reality equate to tension for the reasons Oryx described. Brass spring back etc.

Is it a DPMS lower by chance?

Gotcha. I haven't heard the term interference fit before. Learn something new everyday. It is an Ascend Armory lower. They are no longer in business.

@Oryx Thanks for the detailed explanation. I don't think I am experiencing setback, but I will take my calipers with me and measure the next time i'm able to shoot it. My brass is annealed after every firing so that should for the most part remove the substantial spring back many see.

I have a few things i need to test the next time I am able to get the rifle out. Hopefully some evening this week. I'll report back with my findings...

 

[357Max]

Gotcha. I haven't heard the term interference fit before. Learn something new everyday. It is an Ascend Armory lower. They are no longer in business.

@Oryx Thanks for the detailed explanation. I don't think I am experiencing setback, but I will take my calipers with me and measure the next time i'm able to shoot it. My brass is annealed after every firing so that should for the most part remove the substantial spring back many see.

I have a few things i need to test the next time I am able to get the rifle out. Hopefully some evening this week. I'll report back with my findings...

The reason I asked about the lower. Some of the dpms lowers were known to have low mag catches. This could result in slamming the bullet into the feed ramps causing set back.

Hopefully adding .001 to neck tension/interference fit will resolve it.

 

[mkrem]

The reason I asked about the lower. Some of the dpms lowers were known to have low mag catches. This could result in slamming the bullet into the feed ramps causing set back.

Hopefully adding .001 to neck tension/interference fit will resolve it.

Roger that.

Truth be told, as i mentioned in my original post, this barrel did this a few years ago after a high fire rate which probably heated the barrel up substantially. It makes me feel like something might have happened to the barrel in that instance that is causing this issue. No shit, at 300 yards, I couldn't hit the center of a 5 acre lake after having multiple poa/poi shots.

 

[Oryx]

Have you checked your optic? The inability to hit a 5 acre lake from a 100 fps velocity change (or even a 5" off call shot) isn't going to explain sub MOA to 10 MOL (Minute of Lake)!
You might have multiple unrelated things going on?

 

[mkrem]

Have you checked your optic? The inability to hit a 5 acre lake from a 100 fps velocity change (or even a 5" off call shot) isn't going to explain sub MOA to 10 MOL (Minute of Lake)!
You might have multiple unrelated things going on?

Yes sir, I changed optics even. Unfortunately that wasn’t the issue.

I considered that my bullets were falling apart coming out of the muzzle but I don’t think that is likely.

 

[Oryx]

You could be on to something, but it may not necessarily explain the velocity swings
Bullet separation could be a factor in the disappearing shots on the lake or possibly even a shot that goes way off call as it starts to separate. You may even see it on target if that's the case. Separation can happen with high velocity and twists that will overspin the bullet's design limits.
It can happen but it's not all that common. What bullets were you shooting?
Did this happen with 77's recently and 55 gr a couple years ago?

 

[mkrem]

This happened with the 55 gr Sierra blitz sp a few years ago (lake shooting).

Recently, is where I saw the velocity variation and that was with 77 gr smk. If I’m getting separation with these, I would think it has to be due to the barrel some how.

I just have a hard time believing it would be because the load was too hot when I wasn’t having pressure signs with either load. The barrel is 1:8 so it should stabilize both those projectiles just fine.

I will probably just buy a new barrel and see what happens.

 

[mkrem]

Update: I got a new barrel from BA. They stated that the barrel i sent in was not concentric. They didn’t elaborate any further than that but I assume they meant the bore. Maybe after it heated up, it was causing destabilization of the bullet.

I hope to be able to mount the new one and test it out in the next few weeks.