308 OCW results... Hmmm.

[fredreded]

Rifle: DPMS LR-308 24" factory bull barrel
Brass: Winchester (new)
Bullet: Hornady 178 AMAX
Powder: Varget
Primer: CCI

Just finished running a five strings of five for an OCW workup after switching from 168 gr to 178 gr, both AMAX. All charges were individually weighed.

I started at 43 and worked up to 45 in .5 grain increments. Things were going along as usual until I looked at the chrony data. I have done this a number of times before but I have never received such odd velocity increases. The velocity range in each string varied from around 30 -40 fps. Here are the average velocities:

43gr - 2470
43.5gr - 2536
44 gr - 2588
44.5 - 2602
45 gr - 2597

At the top end, I began to see very slight pressure signs ( shiny spots on the case head) but nothing looking definite.

The best group was the 44 grain one. It was also the one where the velocity seemed to plateau. I don’t understand at all why the top load was actually lower velocity than the next one down. To compound things, I did this test along with another guy who had just received his custom rifle (308 Krieger super match barrel on a surgeon action). His velocities were about 200 fps faster than mine and had pretty consistent velocity increases from charge to charge. (His barrel is an inch and a half longer than mine as well.) My rifle is a pretty accurate rifle with the 168 gr AMAX. I often record groups of 0.8 MOA at 300-500 yards, so I know it is not a crap barrel.

What’s going on here? Further, I am wondering if I can get good performance at 1000 with an average velocitiy of 2585fps. I get pretty different profiles when plugging the data into Shooter (android) and JBM. Both using even G1 numbers say the bullet stays supersonic at 1000 (2000’ altitude).

Thoughts? Thanks.

 

[cali_tz]

Re: 308 OCW results... Hmmm.

you might actually have reached full inside the case at your 44 and subsequent loads are packing the powder. And given the inherent accuracy of the chron it's easy to imagine that the average of the final plateau'd group could be lower than the previous group, as perhaps a low-side error crept in on some of the final group shots, lowering it's average more than actual.

I have witnessed the plateauing too ... it seems that once you get to very near full inside the case, or at least to some total amount of powder, given a slower burning rate, you just can't increase your speed. This happens to me on N570 powder on my rifle with the 250 Scenar.

 

[MALLARD]

Re: 308 OCW results... Hmmm.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: fredreded</div><div class="ubbcode-body">Rifle: DPMS LR-308 24" factory bull barrel
Brass: Winchester (new)
Bullet: Hornady 178 AMAX
Powder: Varget
Primer: CCI

43gr - 2470
43.5gr - 2536
44 gr - 2588
44.5 - 2602
45 gr - 2597

To compound things, I did this test along with another guy who had just received his custom rifle (308 Krieger super match barrel on a surgeon action). His velocities were about 200 fps faster than mine and had pretty consistent velocity increases from charge to charge. (His barrel is an inch and a half longer than mine as well.) My rifle is a pretty accurate rifle with the 168 gr AMAX. I often record groups of 0.8 MOA at 300-500 yards, so I know it is not a crap barrel.

What’s going on here? Further, I am wondering if I can get good performance at 1000 with an average velocitiy of 2585fps. I get pretty different profiles when plugging the data into Shooter (android) and JBM. Both using even G1 numbers say the bullet stays supersonic at 1000 (2000’ altitude).

</div></div>

im not a fan of a 308 for 1000y shooting, its not ideal but allot of people do it , if your gun shoots moa at 500y you can probably do 1.5moa at 1000y , depending on how many shots are in your group. Your velocities seem normal , weird but normal.

you buddy with the kreiger produced 200 fps more using the same load ? thats weird.

you shouldn't use the g1 , try the g7 or g6 , the g1 is inaccurate.

 

[fredreded]

Re: 308 OCW results... Hmmm.

I'm not a fan of using the 308 at 1000 either... If I want to feel good about myself, I use my 338. If however I am feeling like a challenge, the 308 at 1000 is tough for me. I feel like practicing that makes me a better shooter. I want to be able to stay supersonic at that range so I can get some good out of the exercise.

My buddy's barrel is 1 1/2 " longer than mine, that makes up for some of the difference. To add to that, it is a bolt with a hand lapped Krieger. Plus the chamber on his rifle is tighter than the SAAMI spec (95 Palma). All these together I think makes it less strange.

I usually use the G7 but I am finding a huge difference in the numbers with shooter vs. JBM. Shooter has my velocity still at 1700fps at 1000 using G7 whereas JBM only lists 1300. At G1 they are closer together (JBM didn't change much). Not sure what to make of that.

 

[fredreded]

Re: 308 OCW results... Hmmm.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: cali_tz</div><div class="ubbcode-body">you might actually have reached full inside the case at your 44 and subsequent loads are packing the powder. And given the inherent accuracy of the chron it's easy to imagine that the average of the final plateau'd group could be lower than the previous group, as perhaps a low-side error crept in on some of the final group shots, lowering it's average more than actual.

I have witnessed the plateauing too ... it seems that once you get to very near full inside the case, or at least to some total amount of powder, given a slower burning rate, you just can't increase your speed. This happens to me on N570 powder on my rifle with the 250 Scenar. </div></div>

Seeing that someone else has seen this before makes me feel a little better... It just seems to defy the laws of physics.... I started to think that maybe I had somehow slipped into a trance while loading and didn't up the charges...