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Anyone else observe this in gas gun?

Thumper580

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Minuteman
Oct 20, 2013
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Mechanicsville, VA
I hand load for my 308 gas gun.. If I seat the bullets .010 off the lands and handfeed them it shoots knotholes (1/2"). I load the same thing but seat at 2.8" OAL to feed through the magazine it opens up to 1" 1-1/2". Very frustrating. I measured the ogive depth with the Stoney point gauge. At 2.8 AOL the bullet jumps 85-100 thousandths. I know that huge jump isn't helping....... Anyone else experience this? Anyway around it?
 
Yes they usually shoot better when the bullet has a shorter jump. Probably your bullet choice and load development, picking a jump friendly bullet and starting over i bet it shoots fine at 2.8. How is it with factory ammo?
 
Yes they usually shoot better when the bullet has a shorter jump. Probably your bullet choice and load development, picking a jump friendly bullet and starting over i bet it shoots fine at 2.8. How is it with factory ammo?
Any bullet suggestions? I've tried 155AMAX, 155 SMK, 168 SMK. All do about the same thing. Max OAL in mag is 2.81"
 
What's your neck tension on your loads? Are you using any form of crimp at all?

Just curious because while bullet jump CAN lead to accuracy issues from any AR-platform rifle, if your loads are getting beat up upon chambering or otherwise in the mag, you could be getting a fair amount of bullet setback in the cases or otherwise a lot of inconsistencies in terms of your COAL of your mag-fed, chambered rounds that could be a part of the problem. You can check this, at least in part, by cycling the rifle manually (at the range on a live range, aware of your muzzle direction, etc., etc....SAFETY FIRST) and measuring before and after cycling/feeding to see if you are getting any strange results.

If all that checks, then I agree that perhaps getting a barrel without the long lead might be one way to go. You can also try 168gr AMAX, 175gr SMKs as well to see if you have any better results.
 
I was curious if the bullet moved when fed from the mag.... I loaded a dummy round and measured the OAL. I put it in the mag and dropped the bolt closed. Ejected it and measured. Did it several times. No bullet movement. Can an AR barrel chamber be modified or is it a case of replacement barrel with shorter throat.
 
What's your neck tension on your loads? Are you using any form of crimp at all?

Just curious because while bullet jump CAN lead to accuracy issues from any AR-platform rifle, if your loads are getting beat up upon chambering or otherwise in the mag, you could be getting a fair amount of bullet setback in the cases or otherwise a lot of inconsistencies in terms of your COAL of your mag-fed, chambered rounds that could be a part of the problem. You can check this, at least in part, by cycling the rifle manually (at the range on a live range, aware of your muzzle direction, etc., etc....SAFETY FIRST) and measuring before and after cycling/feeding to see if you are getting any strange results.

If all that checks, then I agree that perhaps getting a barrel without the long lead might be one way to go. You can also try 168gr AMAX, 175gr SMKs as well to see if you have any better results.

I have noticed here, at least in my experience, that opening up and polishing your feed ramps to a mirror finish really helps with this issue.
 
I was curious if the bullet moved when fed from the mag.... I loaded a dummy round and measured the OAL. I put it in the mag and dropped the bolt closed. Ejected it and measured. Did it several times. No bullet movement. Can an AR barrel chamber be modified or is it a case of replacement barrel with shorter throat.

A short chamber/throat could be opened up, but in your case with it being too long...its new barrel time if you choose to go that route!

Just supply your bolt to any number of competent smiths, specify the chamber you want/need based on your preferred load/specs...problem solved.
 
You are usually limited by your box magazine's internal length when hand loading for an auto-loader, which means once you've got excessive throat wear you may need a new barrel.

Sierra bullets are well-known for being jump-tolerant. 2.81 is fairly short as even military M118 LR is spec'ed at 2.830 -.030.

You could mill a slot out of the front of your magazine which will allow longer seating length.

You could try bullets with a "Stubbier" ogive profile seated out to just before leade contact (like Berger 155 boat-tails, VLDs, 168s, and 175s; Sierra 2155 155s, 168s, and 180s; and maybe even 185s or heavier (if your twist is fast enough for the length) -- but they may not fit your magazine.

If you're going the new barrel route you'll need to tell the gunsmith which throat you prefer (as stated above). A 175 throat will accommodate a 168, but it's a little tighter the other way around.
 
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Hopefully not a dumb question, but why can't the barrel extension be removed, threaded end shortened to reduce bullet jump and extension refitted (properly). Couldn't this solve the problem. I know this can be done on bolt guns to get past a worn throat.
 
Anyone else observe this in gas gun?

Hopefully not a dumb question, but why can't the barrel extension be removed, threaded end shortened to reduce bullet jump and extension refitted (properly). Couldn't this solve the problem. I know this can be done on bolt guns to get past a worn throat.

You would also need to change the location of the gas port. The PITA is too large. Get a new barrel. Your smith and wallet will thank you.
It sounds as though you need to do load development again.
 
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Have you ever thought about the fact that when you handload the rounds that the rifle doesnt need to cycle a new round into the chamber? Im not saying bullet jump doesnt have anything to do with the issue your having, but its a whole different recoil impulse. Damn 308 ar's are so picky about handling recoil impulse just right.
 
Have you ever thought about the fact that when you handload the rounds that the rifle doesnt need to cycle a new round into the chamber? Im not saying bullet jump doesnt have anything to do with the issue your having, but its a whole different recoil impulse. Damn 308 ar's are so picky about handling recoil impulse just right.
Elf...are you saying seat to fit in the mag, but load one round in the mag and shoot one at a time? In that case I could continue to seat the bullets out and load one at a time into the chamber.....
 
what you could try is the following:

1.) set up two total targets at 100yards.... one target for bullets that are handloaded with .010 jump,,,,, other target that is magazine feed by magazine length.

2.) mark one of the magazine length rounds on the brass body with an x with magic marker,,,, this will be your "dummy recoil impulse round" that you will NOT fire & will only be used to simulate recoil. it can be a live fire round, but you just will not fire it & will eject this round by hand.

3.) load up a magazine with the "dummy recoil impulse" round that you WILL NOT FIRE first, then an actual magazine length round after that which you will fire out of the rifle.

4.) handload your .010 jump round into the rifle, and then insert the magazine that has the two rounds as described above.

5.) fire your handload .010 jump round at one target, and then fire the magazine round at the other target as described above & EJECT BY HAND the "dummy recoil impulse" round (you will re-use this dummy round for 5 total shots for each target).

6.) repeat the steps above for 5 total rounds for each target and compare the two... you might need to do this twice for two total 5shot groups each to see if it actually makes a difference.

7.) just make sure you don't forget to NOT FIRE the "dummy recoil impulse" round.


this will not take long at all & will tell if you if the jump is actually making a difference or not..... OR if it is just recoil impulse as by handloading the round you are pretty much turning the semi-auto into a bolt action rifle.


Elf...are you saying seat to fit in the mag, but load one round in the mag and shoot one at a time? In that case I could continue to seat the bullets out and load one at a time into the chamber.....
 
Yeah...before I spend more money I need to decide if hand feeding the rounds is that horrible..... It shoots fantastic with bullets seated .010 off. I guess it was the principle of the thing. Thanks for the feedback.
 
I guess I look at it this way.... 9 out of 10 SEMI AUTO rifles will not be able to be handloaded with a .010 jump... that magazine will fuck ya every single time... if you're looking to handload rounds with a .010 jump or less on a semi-auto, then you might as well get a bolt action rifle.

also, you need to keep in mind bullet concentricity & the crazy amount of inertia the bullet goes through when it is slammed into the chamber by a semi-auto rifle = just short of a bullet puller.... especially when you do not crimp the bullet.... the seated bullet goes through some insane forces when slammed into the chamber.. put it this way, it's not ideal when it comes to precision shooting. It is not like baby'd handloaded rounds of a bolt action.

due to the above issues of bullet jump, keeping bullet concentricity, bullet pulling inertia of the round being slammed into the chamber, and recoil impulses of the next round being feed into the chamber = why it is hard to out shoot a bolt action rifle in comparison to a semi auto action rifle..

I have found out over the years, as the hotter / bigger the caliber / more recoil the semi-auto has = the harder it is to get really good groups with a semi-auto... trying to get sub-moa with a .223 or 260 is one thing..... trying to get sub-moa with a .308 or 300wm is a whole next step of being ultra consistent with your shots.


Yeah...before I spend more money I need to decide if hand feeding the rounds is that horrible..... It shoots fantastic with bullets seated .010 off. I guess it was the principle of the thing. Thanks for the feedback.
 
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Got a super accurate bolt gun. I tested the bullet movement with bolt closing on gas 308. Bullet doesn't move when chambered. I was just frustrated with the excellent groups when manual loading the gas gun vs using the mag. Obviously the hardware and load is happy, just does not like jumping.
 
Got a super accurate bolt gun. I tested the bullet movement with bolt closing on gas 308. Bullet doesn't move when chambered. I was just frustrated with the excellent groups when manual loading the gas gun vs using the mag. Obviously the hardware and load is happy, just does not like jumping.

Like I said earlier how is it with match ammo. If it shoots say an inch or better with federal gold medal match it is a load development issue, or technique. They are harder to run when going bang bang bang vs just one bang, then load another round by hand. The recoil impulse is different. Seating deeper than normal really helped my DPMS SASS, it didn't shoot great groups on a regular basis until I seated the AMAX to 2.78 and SMK to 2.775".

The bullets you named off are all good choices my 155 nosler load was 45 varget in winchester brass, 2.8" and shot decent. For 168SMK I used either 42.5 varget or 43 IMR4064 in LC brass and again decent. Both between 1-1.25". But 43.5 varget, win brass, 178 amax @ 2.78" and 42.2 varget, win brass, 2.775" were my best shooters. Not fast but most consistent.

Another surefire one that should be near MOA no matter the rifle is 42.5 4064, LC brass, 175smk at 2.8".

If you are shooting shorter ranges and just want tight groups I might try lighter charges. None of the bullets you named off are really 1000 yard performers anyways, even the 155sierra palma needs to be pushed hard. So a 41-42.5 grain charge of a medium burning powder might give you what you want.
 
BCP... I did ladder tests with 4064 and 168's. From 39.0 grs up to 44.0 grs shot from the mag at 2.8 OAL they were all average... Your groups got better seated deeper?? The jump seems to really degrade accuracy.
 
Did you try changing your powder charge when you went to a shorter OAL? What do your groups look like? are they round or vertical? If vertical and you CAN add more powder without pressure issues then add some more. If they are round try a 175 smk. they are very forgiving.
 
Try a lee factory crimp die, it keeps the length consistent when feeding from the mag and ramping up the feed ramps. Custom chamber with short throats can sometimes cause pressure issues with factory ammo so you wont see chambers with short Free bore in factory rifles. Our Chamber in the GAP 10 is short jumps about 20-30 thou with most factory offerings but I get calls on pressure somtimes so there is a tradeoff.
 
Try a lee factory crimp die, it keeps the length consistent when feeding from the mag and ramping up the feed ramps. Custom chamber with short throats can sometimes cause pressure issues with factory ammo so you wont see chambers with short Free bore in factory rifles. Our Chamber in the GAP 10 is short jumps about 20-30 thou with most factory offerings but I get calls on pressure somtimes so there is a tradeoff.
George...I did a test and dropped the bolt on dummy rounds from the mag. The bullet did not move either deeper or farther. Based on this don't think crimping is needed. All loads are within load data from manuals so pressure should not be an issue. Brass look fine. Wish I could get the accuracy when loaded at magazine length vs single loading at .010" off.
 
George...I did a test and dropped the bolt on dummy rounds from the mag. The bullet did not move either deeper or farther. Based on this don't think crimping is needed. All loads are within load data from manuals so pressure should not be an issue. Brass look fine. Wish I could get the accuracy when loaded at magazine length vs single loading at .010" off.

You might be able to. Have you tried working up a new load at mag length? Any change to a load changes it completely. You can't expect it to perform the same at different seating depths just because the charge is the same.
 
There is a big diff. in 5mph crash, a 40mph crash, and a head-on crash. If the bullet has to much jump the speed that it crashes it the lands will rip the jacket an deform it, not good. some times tighter neck will help, so the bullet dose not jump-stop an jump again.
 
George...I did a test and dropped the bolt on dummy rounds from the mag. The bullet did not move either deeper or farther. Based on this don't think crimping is needed. All loads are within load data from manuals so pressure should not be an issue. Brass look fine. Wish I could get the accuracy when loaded at magazine length vs single loading at .010" off.

That test, while a good one is not all telling. The bolt velocity is much faster when you are shooting the rifle.


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I may have missed it, but it looks like you are only checking OAL and not concentricity. Do you have a way to check that? I had a similar issue and found that if I used the Lee factory crimp die and checked concentricity on the Hornady concentricity tool and used it to adjust the bullet if it was way off, that my groups improved....a lot.
 
Elfster your dead nuts on. I have done all these test w a 223 kreiger when single loaded I have a target w 40+ rounds on it all different loads with an average of .410 (3 shot groups) In my personal journey of bolt vs semi I have found the violent semi auto loading process with its variables is really the only reason a bolt gun is more accurate.
 
Another thing with seating depth changes is you can be out of your accuracy node due to longitudinal shock wave and optimal barrel time.

If the load stays the same, but you change seating depth, the start pressure changes.

Try working the load up with the mag-length COL. You can also find mags that allow more COL, or modify 7.62 PMAGS with meplat grooves without blowing through the wall of the magazine.

I would do an OCW test with the 2.810" COL and I bet you find a load that hums.

With that said, I personally will never count on a factory rifle to shoot well for me, let alone be chambered correctly. I had GA Precision build me a .308 AR10 back in 2007, and Jeff asked me what bullet I was going to be shooting right off the bat. 155gr Scenar was the answer, and 155gr Scenar it does shoot extremely well.

If someone is cranking out mass-produced rack-grade barrels that need to be able to chamber all the .308 factory ammo on the market, the leade will have a lot of jump.
 
Different bullets are designed different. Try another brand like Hornady or Sierra or ???. Also if I measure the headspace of 10 fired brass and divide by 10 it gives me the HS of my gun. I then bump the shoulder back the lowest amount I can to make if feed 100%. Thats usually in the .0015-.003 range shoulder bump. Any more HS then that and groups go out the window. At 2-3 thousands I get 1/2 groups or better at 5-7 thousand Im shooting an inch plus and more like 1.5. I think the low HS keeps the round nice and tight and centered. Try it........

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I hand load for my 308 gas gun.. If I seat the bullets .010 off the lands and handfeed them it shoots knotholes (1/2"). I load the same thing but seat at 2.8" OAL to feed through the magazine it opens up to 1" 1-1/2". Very frustrating. I measured the ogive depth with the Stoney point gauge. At 2.8 AOL the bullet jumps 85-100 thousandths. I know that huge jump isn't helping....... Anyone else experience this? Anyway around it?