Speed differences in brass

[Crawdaddy]

Hey all,
I decided to complicate my hobby further by purchasing a chronograph last week. I've got a question about something I've run across while sizing 308 Win brass down to 30 T/C (if you're wondering I want to get ahead of the game if this caliber becomes defunct and as stated earlier, I like complicating things as much as possible).
Anyhoo, after sizing & cleaning the brass I compared volume in a sized 30 T/C case to the resized .308 and found that the .308 holds around 15-20 grains less water than the 30 T/C. I loaded 3 rounds of each brass w/ my proven load of 42.5 gr H4895 pushing a 125 gr Nosler BT.
The results in fps: T/C case 1 - 3008, case 2 - 3008, case 3 - 3005.
Modified .308 case 1 - 3051, case 2 - 3084, case 3 - 3077.
While the T/C cases produced their normal 3/4" group, the modified cases were more like 1-1/2". Do you think this is due to the smaller case volume producing more pressure? If I reduce the powder to get back around 3010 fps do you think my group will improve?
Thanks for any thoughts you can provide and thanks for allowing me to complicate your lives too, feels so good.

[langenc]

Id certainly try it. Im sure others have found that to be true, if so. Makes some sense that the vol would be less in the 'adjusted' case. I guess you are making the case 'smaller' when forming it. The volume you noted is smaller.

[LDBennett]

Measure the once fired 308 brass again for case capacity after you re-size it and trim it (if required). If it is the same as your first measurements before firing then do some testing to get the velocities the same between the TC brass and the 308 brass.

The reason I suggest testing the volume again is that when the 308 brass gets fired it may better conform to the chamber of the gun and change its volume. Until you know for sure that after the first firing the volumes between the 308 and TC brass are actually different, don't change the load, yet.

Accuracy of a gun is effected by the harmonics of the vibrations of the barrel. The velocity of the bullet can change that with all else being the same. So attempting to get the velocities equal might get you the same accuracy (??). But don't jump until you know for sure the volumes are different after the first firing of the 308 brass.

LDBennett

[JLA]

The velocity differences are indeed due to the smaller case capacity producing a pressure spike.

Why dont you just stock up on copious amounts of new unfired .30 T/C brass while you cans still find it?

Seems easier to me than resizing .308 in a .30 T/C die and having to trim off the excess and fireform. Only to end up with a .30 T/C cartridge thats labeled a .308 win and could easily be placed in a .308 win rifle by mistake, which will certianly happen at the most in-opportune time..

[steve4102]

Anyhoo, after sizing & cleaning the brass I compared volume in a sized 30 T/C case to the resized .308 and found that the .308 holds around 15-20 grains less water than the 30 T/C

Really? Typo I hope!

[Crawdaddy]

Thanks for the input guys, sounds like I'm on the right track,. I'll check volume on the brass I fired today tommorow morning, reload it, and see where I stand.
JLA, I don't presently have a .308 so no worries there. Also did you read the part of my post about how I like to complicate things?

[Crawdaddy]

Steve, no typo, why do you ask?

[steve4102]

Originally Posted by Crawdaddy

Steve, no typo, why do you ask?

Well, as the 30 TC has a case capacity of about 54gr of H20 and your resized 308 Win brass has 20gr less case capacity(34gr) that is a reduction of over 30%.

Your H-4895 load of 42.5gr has a load density of 167% case fill. Your 42.5gr of H 4895 (if you could get that much into a case with only 34gr of case volume) and a 125gr Nosler BT would yield an operating pressure of 329,324psi.

[Crawdaddy]

Steve, I musta looked at my data wrong, I see what youre saying. I have no problem at all putting 42.5 gr of powder in the case, comes up right about the bottom of the neck pretty much the same as the regular hornady case. I'll recheck my figures tomorrow when I recheck the fired cases. Maybe my h2o weighs less than yours.

[JLA]

It would have to weight about 1000% more crawdaddy. There should be less than 3-5 grains of H20 volume between the 2. The are very very similar cartridges, only the T/C is slightly shorter with a better shoulder angle.

I did read where you said you like to complicate things. I got a little of that in me myself. I took a bunch of 8mm mauser brass and made .308s out of them and loaded them up and gave them to my wifes cousin for the spanish mauser I gave him, which was bored out and sleeved to .308 win from 7X57 mauser. He shot them all up and then went and bought a box of 8mm mauser ammo and none of it would fit in the chamber, too long. I neglected to tell him what i had done because I can spot the difference between the 8mm mauser and a .308 without looking at the headstamp and to me it was an easy ID, but i work with firearms all the time and sometimes I dont realize others dont.

So as i grow older and wiser i find it best to keep things as simple as possible. for safetys sake if for no other reason at all.

[H-D]

as a side note when I use a "parent cartridge" I color the cartridge red with a sharpie and mark the box it goes in

[Crawdaddy]

1000% exactly JLA, the difference is 1.5-2 gr not 15 -20. Good catch Steve.
You know, I think someone needs to come up with a way to re-headstamp brass. Ill get right on it, Im sure I can make it so simple my kids can use it.
Seriously thanks for the advice guys, especially on the safety front. I get bored sometimes with the status quo but I've got my kids to consider so keep dragging me away from the line before I get close enough to cross it.
I didnt get to check volume in the fired brass this morning, the pillow was calling, but I'll report back mt findings.

[JLA]

shoot, i got it! just use military headstamp, it doesnt have a specific cartridge designation.. just a manufacturer code and date. be perfect for what youre doing with it.

[steve4102]

I loaded 3 rounds of each brass w/ my proven load of 42.5 gr H4895 pushing a 125 gr Nosler BT.
The results in fps: T/C case 1 - 3008, case 2 - 3008, case 3 - 3005.
Modified .308 case 1 - 3051, case 2 - 3084, case 3 - 3077.

I was going to run your case volumes through QL, but I ran into a snag.
What barrel length do you have?

According to Hodgdon(24 in barrel) 125gr Nosler BT with 43gr of H-4895 is Min with and average velocity of 2895 fps. Your load is 1/2 grain below Min and is producing velocities over 3000fps which is very close to Max speed for that bullet.

You sure you are only using 42.5gr of H-4895?

[JLA]

I was kinda thinkin the same steve. Cant be a longer barrel. the .308 is optimal in a 26" tube and actually loses velocity in barrels longer than 27". I cant imagine the .30 T/C being any different.

[Crawdaddy]

JLA, great idea on the mil brass, youre right, should be perfect for this app.
Steve, its a 22" barrel and I was thinking same as you on the velocity, doesnt match the hodgdon data either. I double checked all I was doing with my new chrony f1, made sure it was folded all the way out, 10 feet from the muzzle, all that good stuff. Dont know what else to tell ya unless I can borrow my buddy's rcbs chrony to double check the numbers.
I got around to measuring capacity on the fired brass before I left for work and after FL sizing, trimming to 1.91", and cleaning the difference in capacity shrank some, down to about 1 gr on average. What really surprised me was the range was much smaller on the .308 brass than the original 30TC, .3 gr range vs .8, so I was pretty happy with that.

[Crawdaddy]

Steve, I did typo the load, sorry. Its actually 46.5gr H4895. I gotta tell ya, you guys give great feedback with bad info.

[JLA]

Some of us been tinkerin with this long enough to see the red flags when they pop up without the aide of a manual. Heck steve just saved me on a misprint in another thread not 2 hours ago. Sometimes my fingers go a little faster than my brain does and ill leave stuff out. We got a good tech crew here. Best on the net IMO.

[Crawdaddy]

Yep, I'd have to agree JLA. I was reading the other thread youre talking about, like how you guys work together.