Am I crazy? .308 hits 8" high?

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2TransAms

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So...yesterday I was shooting my Savage 10FP and some friends and I were getting nice little groups at 100 yards with Georgia Arms 168 gr BTHP. We ran out of that after a while and I didn't want to break out the Hornady for my friends (actually,they weren't willing to pay me for it:neener: ). So we switched to some Georgia Arms 150 gr FMJ,and I couldn't hit the paper.

"Where are my rounds even going?" :confused:

So I trekked out to the target,and they were hitting 7-8" higher than the other ammo! Is that normal? Everyone swears that they did not touch the scope,and I'm sure that's true.

But what's odd is that when I got this rifle,I shot the 150's to see how the rifle handled,then when I switched to the 168 gr rounds they only hit 2" lower than the "plinking" rounds. My scope has stayed dialed in for the match ammo ever since,then when I go back to the plinking ammo yesterday it hits 8" higher!? I'm befuddled. Should there be that much difference?
 


Sounds like one of two things:

  1. Your rifle doesn't like those 150 grain loads.
  2. The 150s are hotter than the 168s by quit a bit.

Normally a hot light load will hit lower, not higher, as there less time for recoil to affect the trajectory before the bullet exits the barrel. This can be very noticeable in handguns. I'd go with No. 1 on this.
​
 
Hmm. I thought a lighter, faster bullet would hit higher. One of the mysteries of the universe,I suppose. Thanks.

I guess I'll stick with match ammo...that's why I got the rifle in the first place!
 
Is it possible the barrel twist rate is compatible with one weight and too much/not enough to stabilize the other round?
 
I don't doubt it. I went thru the same thing when I switched between Winchester and Remington. I was so freaked out when my bullets dropped a foot from one brand to the other. Now all I shoot are CoreLoks.
 
Hmm. I thought a lighter, faster bullet would hit higher. One of the mysteries of the universe,I suppose.
Click to expand...

Faster bullets exit the barrel before it has time to rise, causing them to hit lower.

I'll never forget how confused I was when I discovered that my .357s shot way high, like 6", going from 125 to 158 gr bullets.
 
less time for recoil to affect the trajectory before the bullet exits the barrel.
Click to expand...
:scrutiny:

Care to explain how recoil affects the bullet trajectory?

The rifle does not move from the force of recoil until the bullet exits the muzzle.

2400 fps = 28,800 inches per second
Given a 20" barrel, it only takes the bullet 0.0006 seconds to clear the muzzle. Humans can't even flinch that fast. (Flinches affect POI because they happen before the primer is ignited.)
 
Whether they are shooting high is irrelevant, bullet weight or powder strength. The important issue is "did they group well 18" higher?"
 
Lighter/faster bullets hit high because they are still on the way up compared to heavier/slower bullets which generally start to fall (rise less) sooner. Over simplification, but recoil has little/nothing to do with it.
 
Heavier or harder recoiling bullets can indeed hit higher than lighter or less recoiling bullets. This is quite common with handguns but I have not heard of it happening in a rifle. If the muzzle rises only a few thousanths of an inch while the bullet is still in the barrel it could translate into a couple of inches at 100 yards. Not sure this is the cause of your situation but it is possible.
 
rino451 said:
Lighter/faster bullets hit high because they are still on the way up compared to heavier/slower bullets which generally start to fall (rise less) sooner.
Click to expand...
:confused: A lighter,faster load will drop less at 100 yards in a vacuum,but through air a heavier round will maintain velocity better at longer ranges...right?
taprackbang said:
Moral is; "When in doubt, change ammo."
Click to expand...
And that's what I'll do!
 
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Given a 20" barrel, it only takes the bullet 0.0006 seconds to clear the muzzle
Click to expand...

Actually, it takes QUITE a bit longer...because the bullet doesn't attain its' full velocity instantaneously. There are several milliseconds of initial powder burn during which the bullet is being accelerated and the rifle starting to move. It isn't as pronounced in rifles as pistols, but still does happen.
 
Lighter/faster bullets hit high because they are still on the way up compared to heavier/slower bullets which generally start to fall (rise less) sooner. Over simplification, but recoil has little/nothing to do with it.
Click to expand...


Um WOW! I mean just WOW!

figure1x.jpg
 
'Without a chronograph, this thread is worthless!'

Something like that.... ;)

Anyhow, there's a number of things that would need to be quantified before coming to any conclusion. Included are muzzle velocity and the BC of the particular bullet used.

I've had some 150s hit higher than some 180s and some others hit lower.
 
I agree that a chronograph and a bit of study is needed.

My attempt to measure the time out the chute was elementary at best, but still better than a guess. I actually thought of trying to figure out primer strike, powder burn to full ignition, and acceleration, but I didn't think anyone would care about going that far. Little did I know that it
Actually, it takes QUITE a bit longer
Click to expand...
.

The point I'm trying to make is that the recoil of a rifle has little to no impact on the POI downrange. I even sight-in hunting rifles touch them as little as possible, and just allowing the forearm to bounce on a well padded surface.
 
It has been my experience with rifles and pistols, in general, that lighter faster bullets will shoot to a lower poa than heavier bullets.
 
There's no way on earth that the trajectory would cause one load to shift that much at 100yds. Well, there is a way but it would involve a far reduced load. Undoubtedly, barrel harmonics are at work here. Your barrel is twisting like a snake as the bullet is hammered through it and the bullet is leaving at a different point in this twist. You may have some contact between to fore end and barrel which is making the jump worse.

And yes, how were the groups with each load?
 
Lighter/faster bullets hit high because they are still on the way up compared to heavier/slower bullets which generally start to fall (rise less) sooner. Over simplification, but recoil has little/nothing to do with it.
Click to expand...

Amen. Except it should be heavier will hit higher (that's the general rule anyway). I think that's what you meant. Depends on where your target is, but typically with rifles @ 100/200, heavier will shoot higher. Pistols at close range is the same thing.

It's about the arc/trajectory of the bullet.
 
For what it's worth it's been my experience that a lighter bullet with the same load will shoot lower froma handgun, but higher with a rifle.
You are comparing 2 laods that probably have more differences than just bullet weight.
A heavy bullet load will usually be moving slower than the lighter bullet, but not in every load. Commercial ammo can be vastly different even using the same manufacturer.
A good way to check is to get both types of ammo again and test them through 2 or 3 different rifles.
Commercial ammo usually lists a velocity on the box. Even though they are usually optimistic it is a gener way to compare ammo.
Bullet shape has a lot to do with POI too. The BTHP is match ammo where the fmj sounds like milsurp ammo. They are not the same, even if they were the same weight.

Jim
 
plinky said:
There's no way on earth that the trajectory would cause one load to shift that much at 100yds. Well, there is a way but it would involve a far reduced load. Undoubtedly, barrel harmonics are at work here. Your barrel is twisting like a snake as the bullet is hammered through it and the bullet is leaving at a different point in this twist. You may have some contact between to fore end and barrel which is making the jump worse.

And yes, how were the groups with each load?
Click to expand...
Well I thought that even though the bullets drop at the same rate (gravity),the faster load obviously will hit higher because it gets to the target sooner,regardless of bullet weight. That simple math only allows for velocity though. Drag and BC plays a part as the range gets longer,but I'm not smart enough to do all that math in my head,if at all. :)

Anyway,the groups with match ammo were easily 1 moa if I did my part,the 150 grain ammo (which,again,was Georgia Arms reloads) held about 1.5" groups,and you could see the string running all the way down to the bullseye as I kept dropping the crosshairs.

I posted a report about this ammo elsewhere. This is just the first time it ever jumped that much when I switched rounds.

And thanks for the help everyone!
 
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Thank you for the graph RecoilRob.

Obviously I was wrong about movement before bullet exit.

2 milliseconds of total time before the bullet exits. That is even shorter than my calculation.
 
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