Home Forums Reloading Big Game Rifles Neck tension and muzzle velocity?

Recently switched to neck sizing with lee collet die with my .270 win and using the Redding body die. Was previously getting 2950fps with 140 Accubonds out of a 22" barrel using brass full length resized Norma brass with minimal shoulder bump. Shot this morning and got around 2880fps. Thinking I'm at less neck tension with the neck sized brass. Did't change anything else. Think decreased neck tension alone can account for this?

Accuracy was acceptable with new loads, but would like to get back the velocity.

Sooo, wondering if I should just go back to the full length resize or maybe bump up my powder charge to get back to 2950fps.

Thoughts?

That could possibly lower start pressure. You've changed dies, so dimensions of the case are most likely different.

If you want to continue to use that die, I'd bump the charge. Velocity loss indicates lower pressure if all the components are the same. But, I'd let accuracy get first dibs. I'll take an accurate 2880 over an inaccurate 2950.

You could also try partial full length resizing with the original die. I use a .025" shim on top of the shellholder, raise the ram, screw the die against it, lower the ram and give the die a quarter turn. I then raise the ram against the die (with shim still in place) and tighten the lock ring of the die. This squares the die with the shellholder and doesn't bump the shoulder back.

I anneal and full-length size every 3rd to 4th loading, when the bolt tells me things are getting hard to close.

One last thing is about the chronograph itself. Would light conditions allow a consistent reading?

Could be neck tension, or something else such as barrel and ammo temperature, lighting conditions or normal random variation.

Take 5 of the new rounds and 5 of the old rounds to the range. Alternate shooting one from each batch, allowing the barrel to cool well between shots, recording MV vs. new/old. That will give you an indication of whether you've really lost velocity or not.

Then, if you find something, you can worry about what caused it.

Right now you could well be chasing phantoms.

I think that is a very good explanation of what we do each time we reshoot a load and look at the speed and group size compared to previous sessions.

Thanks. I'll try that.

Same conditions this week as last week. Only changed variable was neck sized brass with body die bumped back 0.002 vs prior brass being full length sized with 0.002 shoulder bump. Velocity measured on the new LabRadar. Has been solid up to now.

That pretty much does away with light conditions as a possibility.

Unless, the original velocity was obtained on another chronograph.

Neck tension will have little effect on velocity. Necks are the first thing to expand at powder detonation.

What I think has happened is that in full sizing cases you were reducing the internal volume of the case. When you neck size, the volume of the powder chamber is larger which reduces the pressure therefore the velocity.

I did some tests on new brass versus reloaded brass. In a 338 Win Mag case there was approximately a half grain difference in the powder between new versus reloaded cases. That slight velocity difference affected accuracy. I had to add a half grain to my accuracy loads if I was either fireforming new cases or otherwise using when accuracy was important. I only neck size and use fired brass for important stuff.

That slight difference in velocity could also be differences in temperatures at the times you shot.

Yes, Vic is right. That does rule out the effect of variation in light.

Do the 10 shot test, and PM me the data. I'll do the stats for you.

All good thoughts.
Vic,
Yep. All velocities on the Lab Radar. 2950ish was pretty consistent with the old load over multiple shooting sessions.
Was shooting twice fired Norma brass. First two firings with FL sizing. Last session with neck/body die combo.

denton,
I don't have any brass prepped with FL sizer at the moment, but will get a few loaded up and shoot the 10 shot test. Might pull a few bullets of the new batch and add a little more powder too. Might be a little bit before I get it done. Hopefully next weekend.

Bob338,
New brass was prepped with a body die as well. Do think that should also reduce the internal volume? Didn't compare water capacity, but that is a good thought as well.

Accuracy was a little better with velocity at 2950, so new loads are probably a no go in their current form regardless.

Thanks Guys!

Everything I have experienced and read would say neck tension has little (i.e. a few fps change) if any effect on MV.

I think the case capacity difference is the issue as others have written. I would measure water capacity/weight of each to compare. I do think neck tension can be a factor in accuracy and possibly velocity. On some pistol loads a crimp can make the load have more uniform accuracy.

Lee collet dies usually have around .002 neck tension. Most factory FL dies have more at .003+ depending on brass thickness. Lee sells smaller pilot rods if you want to experiment or you can polish one down to the desired diameter to test. I think I would keep the original if it turns out to not make a difference the less you work the brass the longer it will last.

Also work hardening will affect neck tension. I have heard of people who anneal every or every other loading for this reason.

I would probably just bump the load up slightly if it was not already redlined.