Affect of speed on accuracy for .308?

Snakum

Snakum

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Feb 21, 2010
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#1
I started loading last year right after getting into long range shooting, and am just picking it up again after not shooting much for the last six months. Working up a load for my semi auto the best accuracy came from a round that only runs 2437 fps avg in the 18" barrel. I was told this is a problem and that the 308 needs to run hotter than that for best performance.

Is this correct, in your experience? Does slow speed affect accuracy? Or only performance in the wind?

If true, at what fps min should my semi loads be running?

Hotter/heavier loads don't shoot well in this rifle so I wound up running 155gr Amaxes over 41.5gr IMR4198. If I run them any hotter the groups open right up. Should I try a much lighter bullet that I can send faster? Something like the 135gr SMKs?
 
#2
Re: Affect of speed on accuracy for .308?

Rule of thumb usually is a projectile with a decent BC going no slower than 2500fps for shooting at 1000yds. How far do you plan to shoot? I would just try different powders. Also what bullet?
 
#4
Re: Affect of speed on accuracy for .308?

Sounds a bit slow for a 155gr class bullet, even in an 18" barrel, i would think you would get 2600fps easy. Have you tried varget, RL 15, or even IMR 4850? All good powders for the .308. You might have more accurate loads with a higher velocity with those powders, there's no definite way to tell as you know every rifle is different. Or you could take your current load and work it up in .1gr increments if you want it faster.

That being said looking at JBM it will still be supersonic out to 700yds so it should be fine IMO.
 
#5
Re: Affect of speed on accuracy for .308?

In my 19" bolt gun I am running the 155 scenar over 2900 fps. You need to bump up your load and/or change powders to get more velocity and you will probably find another accuracy node. CKA runs his gasser at 2850 with the 155 AMAX if I remember correctly.
 
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#6
Re: Affect of speed on accuracy for .308?

Ive always liked 4895 for the burn rate, it's pretty fast and has been consistent for me. Im prepping some cases now (my only day off) and will go ahead and bump it up some more. Groups start opening up around 42 gr with this rifle but I can try again to see exactly how far I can go before losing accuracy.

Thanks for the info.
 
#8
Re: Affect of speed on accuracy for .308?

IIRC Varget is less temp sensitive but burns slower than 4895, correct? And RL15 is very close to what Federal is using now for GMM ammo? Which powders burn faster than 4895?
 
#9
Re: Affect of speed on accuracy for .308?

"Does slow speed affect accuracy?"

Sure, in that velocity/twist rate and barrel time both affect accuracy. Actually, common bullets are well stabilized at common velocities/twist rates so that really isn't a major consideration.

Barrel vibrations affect accuracy and controlling the exit time of bullets is critical to where the barrel is pointing when a bullet exits. Timing the exit depends on the speed of bullet travel and getting that right is the real goal during load development. The fact that not all barrels vibrate at the same frequency or amplitude is why there is no magic bullet speed or pet load that can be depended on.

Wind and the travel time down range are the only things that directly matter at long range, pure accuracy at any range is far more critical than velocity alone. If you're getting good velocity now, ignore all the 'expert' help, your bullet will get there well before it hits the ground!
wink.gif
 
#10
Re: Affect of speed on accuracy for .308?

There are a number of factors involved, here.

A) there are a number of accuracy nodes for each weight of bullet* for 155s in a 24" barrel there are at least 4 accuracy nodes. In between the accuracy nodes, accuracy decreases for a while, then, bango it falls off a cliff. These are called scatter nodes.

B) semi-autos are a little harder to find that "one good load" because you are looking for two effects to become correct at the same time. You must find an accuracy node AND you must find a powder that has the right amount of pressure when the bullet opens the gas port. This is significantly harder than for bolt guns, and you may have to try several powders to find a combination that is optimized.

C) 155s in 308 from an 18" barrel should be able to leave the muzzle at close to 2800 fps. We shoot them at 2950 fps from 24" tubes with sane pressures.

D) when you say hotter loads don't shoot all that well, is this simply accuracy, or does that load cycle the action improperly (heavily). If so, you might be looking at a slightly faster powder so that the pressure at the gas port is within reason as the bullet passes.


(*) Bullet weight and shank length mainly and a small contribution to jacket hardness.
 
#11
Re: Affect of speed on accuracy for .308?

It's an AK platform, Saiga 308 with a relatively heavy barrel for an AK, but still the barrel and the receiver itself flexes when fired. It's difficult to find a load where the flex of the barrel and the receiver are 'in synch' so that the bullet leaves the barrel at the same point every time. This becomes easier with lower pressure/velocity loads. That said, however, it is possible there is another point like this farther up. So this week I'm going to work up some very hot/fast loads too that I didn't try before, and some mildly hotter loads, and see if there's a sweet spot with better velocity than the 41.5. If that doesn't work I'll try a different powder.

Thanks for all the info. Very helpful.
 
#12
Re: Affect of speed on accuracy for .308?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: blklabs</div><div class="ubbcode-body">In my 19" bolt gun I am running the 155 scenar over 2900 fps. You need to bump up your load and/or change powders to get more velocity and you will probably find another accuracy node. CKA runs his gasser at 2850 with the 155 AMAX if I remember correctly. </div></div>

What barrel are you running? Thats mighty fast for a 19" barrel
 
#13
Re: Affect of speed on accuracy for .308?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: vman</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: blklabs</div><div class="ubbcode-body">In my 19" bolt gun I am running the 155 scenar over 2900 fps. You need to bump up your load and/or change powders to get more velocity and you will probably find another accuracy node. CKA runs his gasser at 2850 with the 155 AMAX if I remember correctly. </div></div>

What barrel are you running? Thats mighty fast for a 19" barrel </div></div>

Yes sir it is. It's a Kreiger 1-11. I get questioned everytime I put numbers up ......
 
#14
Re: Affect of speed on accuracy for .308?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: blklabs</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: vman</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: blklabs</div><div class="ubbcode-body">In my 19" bolt gun I am running the 155 scenar over 2900 fps. You need to bump up your load and/or change powders to get more velocity and you will probably find another accuracy node. CKA runs his gasser at 2850 with the 155 AMAX if I remember correctly. </div></div>

What barrel are you running? Thats mighty fast for a 19" barrel </div></div>

Yes sir it is. It's a Kreiger 1-11. I get questioned everytime I put numbers up ...... </div></div>


These velocities seem very unlikely given the barrel length and powder used. What band and model chrono(s) are you using?
 
#15
Re: Affect of speed on accuracy for .308?

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">155s in 308 from an 18" barrel should be able to leave the muzzle at close to 2800 fps.</div></div>

Not in my semi. Unless I want it spraying like a shotgun and flattening primers. I'll be happy with 2500 fps if it stays accurate.
 
#16
Re: Affect of speed on accuracy for .308?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Snakum</div><div class="ubbcode-body"> Does slow speed affect accuracy? Or only performance in the wind?

If true, at what fps min should my semi loads be running?

Hotter/heavier loads don't shoot well in this rifle so I wound up running 155gr Amaxes over 41.5gr IMR4198. If I run them any hotter the groups open right up. Should I try a much lighter bullet that I can send faster? Something like the 135gr SMKs? </div></div>

I don't know if you are limiting this to the 155 but if not:
Check out some of the posts in the reloading section inre 208AMAX
I've drunk the Kool-aid. 2190fps MV from my 18" DPMS AR10, but it retains more velocity at a 1000yds than a 175SMK
 
#17
Re: Affect of speed on accuracy for .308?

I read about the big bullets. Very interesting. With this rifle, though, I'm only shooting to 500 yards and a heavier bullet would spray even worse. If Hornady made a 135gr Amax I'd be trying that.

After school is done in December and my time frees up again I want to start shooting some F-Class and very long range tactical matches. I might try to work up some heavier stuff for the bolt gun.

This week I'm trying the 155s at 41.8, 42.2, and 42.6 grains now that I'm shooting it much better. Still trying to get it moving faster without opening up the groups.
 
#18
Re: Affect of speed on accuracy for .308?

I ran a bunch more thru at 100 and the 42.6 loads seemed to work best, while still staying under 2" for ten rounds. The rifle seemed to settle down again right around this mumber. I ran ten shots for four strings. In each string three shots stayed an inch or under, four averaged 1.25, and five thru ten never went over 1.75" unless I knew I genked one. At 500 yards and under that'll still work. I'll try to get out to the range next week and get dope to 500 yards. Thanks for the info.
 
#19
Re: Affect of speed on accuracy for .308?

Got to 500 yards with the 42.6gr loads today. It's not too bad. On average I can send ten rounds at 400 or 500 or whatever and 4 will stay around a minute, three or four more will stay under 2 minutes, and a couple will sometimes fly outside that. It'll work though.
 
#20
Re: Affect of speed on accuracy for .308?

I found my old thread while searching for info on the 135 SMKs I never tried. For the record I never got to 500 yards with the 155 Amaxes, only 400, and they did stay right around 2 MOA for ten for a string or two. But I never duplicated that again. At 400 yards they always started spreading out more than 2 MOA after that first day, even though I followed the same recipe to the letter.

But ....

I tried a box of 150gr SMKs over various wts of 4895, from 41.0 to 42.5 and the 41.5gr charge shot the best yet. Far more of each ten round string stayed under 2 MOA out to 300 yards. I shot one group that had five of ten touching or almost touching, and seven of ten at 1.5 MOA. Alas ... two or three more would be in the netherworlds. LOL.

Trying 135 SMKs next week when I get back off the road.
 
#21
Re: Affect of speed on accuracy for .308?

In my experience, speed is irrelevant *with a catch* and accuracy trumps all else.

The catch is this, is the bullet still supersonic to the range you plan to shoot at? That's the only limiting factor. People used to say 308 was only good to 800m. Well yeah, with 168gr projectiles going subsonic at 800m...thats true.

Get familiar with the JBM site and run the numbers on the cartridge you plan to use and the muzzle velocity you get with your load. You can fudge the numbers to figure out where you need to be, increase them till you hit your desired range. For me, 1000 yards is all I have easy access to, there are some places if I wanted to drive I could go farther.

I've shot 168s all the way up to 208s, with the 208s going so slow people have, and continue to balk at the slow muzzle velocity. Even when it retains more energy and stays supersonic farther than just about anything else available.

That said, they are tricky to load and feed reliably in a repeater. I'm transitioning to 178 HPBT projectiles from Hornady because they are supersonic past 1200 yards at sea level and I can load and feed them a lot easier. If you have the velocity to get to the max range you plan to shoot at, then going faster doesn't do anything but wear out the barrel. Finding a good accuracy node is WAY more important than a few hundred, if that, feet per second at the muzzle.

Rich
 
#22
Snakum said:
I started loading last year right after getting into long range shooting, and am just picking it up again after not shooting much for the last six months. Working up a load for my semi auto the best accuracy came from a round that only runs 2437 fps avg in the 18" barrel. I was told this is a problem and that the 308 needs to run hotter than that for best performance.

Is this correct, in your experience? Does slow speed affect accuracy? Or only performance in the wind?

If true, at what fps min should my semi loads be running?

Hotter/heavier loads don't shoot well in this rifle so I wound up running 155gr Amaxes over 41.5gr IMR4198. If I run them any hotter the groups open right up. Should I try a much lighter bullet that I can send faster? Something like the 135gr SMKs?
Click to expand...
Mine likes 42 grains of 3031 and 168 gr. A- max 2700 fps. 3 bullets in the same hole at 100 yards no joke.
 
#23
I am currently working with n550 and 155 gr. Lapua scenars . With 50.2 grains they claim 2956 fps out of a 24 inch barrel. I am running 18" s.s. with muzzlebrake. I am starting at 48gr. And working up. Will post results.
 
#25
Jerrydaniels said:
I am currently working with n550 and 155 gr. Lapua scenars . With 50.2 grains they claim 2956 fps out of a 24 inch barrel. I am running 18" s.s. with muzzlebrake. I am starting at 48gr. And working up. Will post results.
Click to expand...
The only thing slower than a 308 is your 12 year late response to this thread….
 
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