Changing primers

[bh-ltr]

Interested to see what opinions rise from this...

I've been shooting the following load for almost 1K rounds:
308 Win
175 SMK
Lapua Brass
Varget Powder
Federal 210 primer
44.5 grain charge
2.800 OAL
2580 FPS

Because of recent times I've been forced to switch primers. I now have 5000 Remington 9.5's.
In changing only the primer I got 2601 FPS. After changing to 44.3g I got 2577 FPS with a SD of 8 fps.

The original load was found for this rifle using the OCW method. I'd thought that I might go down to 44.3g as that should maintain barrel time and thus keep me in my OCW node. I also like the fact that it should match my current data cards. However being a 20" barrel 308 shooter it seems worthwhile to want to keep the extra velocity.

I also suppose I could rerun a full up OCW but thats kind of a pain I'd rather be shooting drills and longer ranges and need to get some major reloading done soonish.

What are your thoughts? Please vote in the poll.

 

[bh-ltr]

I guess I'll go in and throw my preferred vote in. Something makes me want to chase the additional velocity but the not having to change my dope and being able to still trust my databook is quite attractive.

 

[Grumulkin]

I have some questions and observations:

1. Is the accuracy the same with both powder charges? If accuracy is the same, I wouldn't decrease the load just to get the same velocity.

2. How did you determine there was a velocity difference in the loads? Were the loads comparing the two primers shot on the same day and what was the sample size? I wouldn't expect a 0.2 grain difference in powder charge to make any significant change in velocity. Indeed, the difference between 2601 fps and 2577 fps is only 24 fps which is within the difference that could be explained by shooting on different days or relying on a small sample size for your data.

 

[bh-ltr]

I have some questions and observations:

1. Is the accuracy the same with both powder charges? If accuracy is the same, I wouldn't decrease the load just to get the same velocity.

2. How did you determine there was a velocity difference in the loads? Were the loads comparing the two primers shot on the same day and what was the sample size? I wouldn't expect a 0.2 grain difference in powder charge to make any significant change in velocity. Indeed, the difference between 2601 fps and 2577 fps is only 24 fps which is within the difference that could be explained by shooting on different days or relying on a small sample size for your data.

Sample size for old load was 14 rounds (all I had left). Sample size for new loads was 10 loads for each charge weight. All ammo was fired in a 1 hour span of time over a MagnetoSpeed chronograph. Standard deviations were 14 for the old load (which was done by taking all charges from an RCBS charge master); 10 for the 44.3 load, and 8 for the 44.5 load. Each charge for the 10 new loads were individually balance beam weighed (I believe this accounts for the lower SDs).

I have a feeling that these SDs would have fallen a small amount with larger sample sets. But, I feel that 10 bullets for each load is an ample test for this purpose. Not to mention that the 24fps delta between these loads is 3 standard deviations away if you use the 44.5grain SD (2.4 SDs away if you use the 44.3g SD). That makes it quite clear that there is a true and very real difference. Mathematically it is quite clear that these two data sets are not from the same population. This is a statistically significant difference. (sorry for the lingo I've taken way too many math classes in my day)

Both loads grouped fairly well and similarly. I'd say that the 44.5 was the best group of the two but you'd have to take out one called flier. Even better news is that both groups were in the same location. This leads me to believe that I'm still in a good OCW node.

Here is the shot data from that day...

Old Load Velocities:
2574
2590
2574
2570
2590
2582
2557
2573
2590
2566
2564
2585
2604
2604

New primer 44.3 grains:
2589
2568
2570
2569
2578
2569
2570
2575
2590
2598

New primer 44.5 grains:
2612
2594
2600
2593
2598
2597
2602
2619
2604
2593

 

[Grumulkin]

In that case, I'd stick with the old load, i.e., powder charge, and not worry that the velocity was a little higher. In fact, I'd be quite happy with it.

 

[bh-ltr]

Just for completeness and because I often wonder what people take away from these threads. I've since loaded up 1000 rounds of 44.3 grain bullets. I've gotten a chance to head out to a range where we had 8 targets at varying ranges from 100 to 950 yards. All my dope cards for that DA held true which means that all of my other DA dope cards don't need to be reprinted. I feel like I'm still using my same old primer and I'm a happy boy who gets to shoot at fun shit (rather than doing an OCW test). Granted I think its the best way to get information on the load that fits a rifle and what not but they're boring as shit to shoot! Personally I find dry fire drills more fun.

~Brett

 

[leadbullet]

keep velocity the same and all your range data will stay the same, simple enough.
cheers

 

[bh-ltr]

keep velocity the same and all your range data will stay the same, simple enough.
cheers

Yep that was the path i took.