OCW and velocity with different brass

[akxx]

Once you find OCW for a given brass, is it a general rule of thumb that if you change brass (lower or higher capacity brass) and load to achieve the same muzzle velocity then you are also at OCW for the new brass?

(i.e.: once you have found OCW, just aim for that same velocity regardless of which brass you use in the future?)

 

[CoCaDoRi]

Re: OCW and velocity with different brass

brass varies from manufacturer to manufacturer so will OCW from my experiences.

 

[Niles Coyote]

Re: OCW and velocity with different brass

Some very good reading on the subject

 

[kombayotch]

Re: OCW and velocity with different brass

I generally find it at about the same velocity for different brands off brass. A few fps. less for brass with less case capacity and a few fps. more for brass with more case capacity.

I posted some results with the same load in Lapua and Hornady brass here:
http://www.snipershide.com/forum/ubbthre...602#Post1087602

 

[akxx]

Re: OCW and velocity with different brass

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: kombayotch</div><div class="ubbcode-body">I generally find it at about the same velocity for different brands off brass. A few fps. less for brass with less case capacity and a few fps. more for brass with more case capacity.

I posted some results with the same load in Lapua and Hornady brass here:
http://www.snipershide.com/forum/ubbthre...602#Post1087602 </div></div>

Holy smokes that is some incredible accuracy you're getting!

If my theory holds, then it sure would be nice to only have to do development testing for 1 type of brass and then just 'load up' to the same velocity in other brass types.

What were the differences in weights between the 2 types of brass you used?

 

[kombayotch]

Re: OCW and velocity with different brass

If things are done consistently, there is no reason why you shouldn't have repeatability (with minor offsets). Tracking velocity is what I do. The relationship between it and barrel time seems to be quite consistent. Even for different powders with similar burn rates.

I don't recall what they weigh. Internal case capacity is what matters (measured in grains of water). The Lapua brass has a capacity of 55.7 gr. H2O and the Hornady brass is 57.4 gr. H2O.

 

[MitchAlsup]

Re: OCW and velocity with different brass

I know that going from Win brass to Lapua brass in 308 one should find the same OCW about 0.3-0.4 gr lower in Lapua brass. This should achieve the same MV.

 

[kombayotch]

Re: OCW and velocity with different brass

The difference depends on the bullet weight and burn rate of the powder.

 

[sp388]

Re: OCW and velocity with different brass

Has anyone compared between 308 Federal GMM brass and Lapua brass if both are trimmed to 2.005 length ?

 

[sp388]

Re: OCW and velocity with different brass

Less capacity will equal higher pressure, if the same charge weight is used in each, and thus more speed ??

 

[kombayotch]

Re: OCW and velocity with different brass

I use Lapua and FGMM interchangeably.

Yes, if you put the same charge in two cases with different capacities, the one with less capacity will produce a higher velocity at a higher pressure.

 

[sp388]

Re: OCW and velocity with different brass

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: SP308</div><div class="ubbcode-body">Has anyone compared between 308 Federal GMM brass and Lapua brass if both are trimmed to 2.005 length ? </div></div>

KOMDAYATCH - thanks for the input on this.

Anyone know the H20 volume of these cases after trimming ??? Before trimming ??

 

[kombayotch]

Re: OCW and velocity with different brass

The exact volume will depend on your chamber. Its something you need to measure in your gun (it needs to be measured with a fired case). For my tight chamber, it is 55.7 gr. with Lapua brass at 2.005". It would most certainly be larger in a stock Savage, for example.

The trim lenght isn't that critical. If you're using Quickload, it's one of the inputs and is factored in to the model. If you're comparing two different brands of cases, just make sure they're trimmed to the same length (ideally 2.005").

 

[sp388]

Re: OCW and velocity with different brass

I tried to get the case volume with both- but they were quite different and I found that trying to get the water out into the scale pan (10-10 scale) was very difficult due to a suction effect and the water wouldn't release so i had to use a small allen wrench to try and help it out.

I thought being cleaned, FL sized and and cut to length would then show the difference in case volume due to brass thickness.

I got 176.5 grains empty holding 52.3 grains of H20 for sized and trimmed Lapua and 178.8 empty holding 53.8 H20 for the Fed GMM prepped brass. This sound right? I thought they would be closer as many say that other than Fed brass being softer, they are otherwise similar.

 

[sp388]

Re: OCW and velocity with different brass

i just check the ones I fired yesterday. These are now twice fired cases.

Lapua was 176.0 empty and held 54.0 H20 and was 2.007 oal from 2.007 trim length.

The Federal was 183.3 empty and held 53.6 H20 and was out to 2.008 oal from 2.007 trim length.

 

[kombayotch]

Re: OCW and velocity with different brass

What kind of chamber is this?

 

[sp388]

Re: OCW and velocity with different brass

Rifle has been rebarreled and blueprinted with shilen select match. Chamber is supposed to be standard although it was supposed to have been shortened (not tightened) so that there would be a shorter jump while still fitting a remington magazine that was 2.840 max. I don't know exactly what the chamber length ended up at in the end but I have switched to the AICS and now have a much longer mag and near as I can tell I load the SMK to 2.898- 2.900 (3.253 sinclair ogive nut)to kiss and the Scenars 2.905 to 2.907 (3.246 ogive with sinclair nut) to be maybe 1-3 thou in the lands. This is still a work in progress as I am new to reloading and measuring this stuff. I often times get measurements different from the others. ?? ..

 

[akxx]

Re: OCW and velocity with different brass

I've got FGMM brass with 2 different "nodes" of mass:

Lot 1 ranges from 172-174 grains
Lot 2 ranges from 177-179 grains

How's that for throwing a wrench into the FGMM/Lapua similarity comparison?

 

[Grump]

Re: OCW and velocity with different brass

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: akxx</div><div class="ubbcode-body">I've got FGMM brass with 2 different "nodes" of mass:

Lot 1 ranges from 172-174 grains
Lot 2 ranges from 177-179 grains

How's that for throwing a wrench into the FGMM/Lapua similarity comparison? </div></div>

All mine's at the light node. Much lighter than LC Match.

 

[sp388]

Re: OCW and velocity with different brass

Anyone else --- CASE Volumes ????

 

[Short Round]

Re: OCW and velocity with different brass

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: SP308</div><div class="ubbcode-body">I tried to get the case volume with both- but they were quite different and I found that trying to get the water out into the scale pan (10-10 scale) was very difficult due to a suction effect and the water wouldn't release so i had to use a small allen wrench to try and help it out.

I thought being cleaned, FL sized and and cut to length would then show the difference in case volume due to brass thickness.

I got 176.5 grains empty holding 52.3 grains of H20 for sized and trimmed Lapua and 178.8 empty holding 53.8 H20 for the Fed GMM prepped brass. This sound right? I thought they would be closer as many say that other than Fed brass being softer, they are otherwise similar. </div></div>


If I was going through this routine of getting water weight for volume I'd weigh the brass empty, write the number down, then fill with water and then reweigh. Then mathematically find the weight of the water through subtraction. Too much error in dumping the water and leaving droplets behind.

It's called taring or a tare weight.

I hope this helps.