Are these tumbling? 225 gr Hornady BTHP's in 300 WM

[cowboyarcher]

Hi Fellas,

Finally got out to shoot my new to me Browning A-Bolt in 300 Win Mag. I had hopes of making this a good, light weight, mid to long range hunting rifle so when I found a screaming deal on some 225 gr Hornady BTHP's I jumped on them and loaded them over 75, 76 and 77 gr of Retumbo and headed out to test them out. COAL was 3.461" average and they were loaded in 1x W-W Super cases w/WLR primers.

I was shooting at the lower left target (8.5x11 paper each) and you can see what appears to be impact from debris (first shot was a big puff of dirt in front of the target) and then two long holes in each of the right targets. Are these tumbling and not stabilizing? I only shot the 3x of 75 gr given what I saw on target.

The only other load I a had on hand was a 150 SGK over a light load of 760 just to sight with. They printed normally and in about a 2" group.

Thanks for the help as always and God bless you all,

Adam

Here are some photos:

 

[smokeumm]

What is your barrel length, and the barrel twist rate? I would think a 26 inch barrel with a twist rate of 1:10 should be good enough. Might be smart to shoot one shot at 25 and another at 50 yards to look at the angle of the impact.

 

[Sully2]

If you have a hole...thats anything but ROUND.....you have some amount of tumbling!

 

[cowboyarcher]

22" barrel and 1:10" twist.

I did not shoot the 6 rounds I loaded heavier, but how will driving them faster or slower affect stabilization?

 

[westcliffe01]

Stability is RPM dependent. The actual RPM's can be achieved by various means, such as shooting at a lower speed through a faster twist rate, or a higher speed through a slower twist rate.

For a given twist rate, a higher MV will provide increased stability. The Berger stability calculator will show that. However, the difference between a 20" barrel and a 28" barrel should not make that much practical difference to stability, although it will make a difference to downrange energy.

For grins, I ran some numbers on the Berger stability calculator. Hornady does not state the bullet length, nor do they recommend a twist rate (kinda hard to believe from a premier bullet maker), so I looked up the length of a Berger target bullet of a similar weight (1.6"). I then ran the calc as follows:
Caliber 308
Weight 225gr
bullet length 1.6"
Barrel twist 10
Muzzle Velocity 2500fps
Temperature 20F
altitude 0feet

The calculated SG was 1.34 , for 60F and 1000ft it was 1.5 1.5 is "comfortable stability" and you should really do better than 2400fps unless you are being handicapped by that short barrel.

"corrected base cartridge"

 

[cowboyarcher]

I'll try the Berger calculator w/ my variables, I've never tried it. But I have no experience w/ bullets this long either.

Here are the impact holes, you can see the 2" group w/the 150s in the first also:

 

[cowboyarcher]

Caliber 308
Weight 225gr
bullet length 1.6"
Barrel twist 10
Muzzle Velocity 2500fps
Temperature 70F
altitude 8000 feet

I get a SG of exactly 2 with those inputs, so not sure what is off. . .

 

[Stanm70]

Is it possible you were not completely square to the target? I know its a long shot but maybe.

 

[westcliffe01]

You increased both temperature and altitude, each which make the air less dense and thereby increases stability. The reason I put in 20F and 0ft initially is that is a pretty tough case for stability with cold dense air. Like what we have in the midwest in winter.... Here we have to be careful developing loads in summer which may not work out at all in winter...

 

[cowboyarcher]

You increased both temperature and altitude, each which make the air less dense and thereby increases stability. The reason I put in 20F and 0ft initially is that is a pretty tough case for stability with cold dense air. Like what we have in the midwest in winter.... Here we have to be careful developing loads in summer which may not work out at all in winter...

Ah, I see what you were getting at. So even under very tough conditions this bullet should be stabilizing in this barrel. 8000' and 70 degrees were the actual conditions so it should have been even easier to get the bullet stable.

Impact holes are oriented different from each other and they would have impacted at least 3 feet apart. Seems odd given the group from the 150's.

Are there any other variables to consider?

 

[westcliffe01]

You are positive that you were not hitting low and having the bullet ricochet up at the target ? You don't mention the shooting distance, but I would expect a 225 to hit lower than a 150 at short range simply due to the time of flight being longer and it would therefore be dropping more. Whats the difference in MV ? Should be at least 300fps right ? At long range the 225 would retain a lot more energy and would end up going faster than a 150 and thus would actually drop less at the end.

 

[cowboyarcher]

I suppose it's possible they were hitting the dirt, but there was only the one dust cloud on the first shot. I suppose I should shoot a group at 25 and 50 just to double check. These were fired at 100 yards.

 

[westcliffe01]

It looks like the 225 would take 28% longer to get to the target at 100yds compared to the 150. That is a difference of 27msec (95msec for the 150 and 122msec for the 225).

From a perfectly horizontal barrel, using Newtons law S=1/2*G*t^2
The 150 will drop 44.2mm and the 225 will drop 73mm.

So a difference of less than 30mm or about 1.5" So that does not sound like your issue.

 

[feelinducky]

This may sound out there but I had a target look similar. I was shooting a 45-70. I had printed the targets off my printer at home on standard thin copy paper. The cardboard I has stapled to had been shot out pretty good. When the bullet went through the paper it would get some tearing and it looked like it was keyholeing but the paper was thin and it was actually tearing around the bullet hole. I can't tell if you printed your own target or not or the condition of the cardboard backing. I hope this helps you.