POI and Velocities Changes with 1-Fired Brass

[AM_Hunter]

Attention Reloaders of Rokslide!

Is it typical to see a POI shift and Velocity increase going from virgin brass to once-fired brass?

I was out shootings and testing velocities on my .308 the other day and had about 13 loaded round with brand new hornady brass. The remainder of my rounds were all once-fired, this is from the same gun, same load data, bullet, everything.

For the once-fired brass I full length sized it, cleaned it, chamfer and deburred, I did NOT however trim or anneal this brass. Dont know if that would make a major difference. I had been crimping all my .308, new brass and once-fired, because I did not know any better but I dont plan to crimp anymore.

First 5-round group virgin brass
Average: 2605.4 FPS
Ext Spread: 25.9 FPS
STD Dev: 8.5 FPS

First 5-round group once-fired brass
Average: 2613.5 FPS
Ext Spread: 115.2 FPS
STD Dev: 41.3 FPS

POI shift right about 1"

Second 8-round group virgin brass
Average: 2602.1 FPS
Ext Spread: 31.3 FPS
STD Dev: 9.7 FPS

Second 5-round group once-fired brass
Average: 2635.0 FPS
Ext Spread: 22.7 FPS
STD Dev: 8.0 FPS

POI shift right about 0.75" - 1"


As you can see theres a slight increase in velocity on the first round of shots but the once-fired in that group had a pretty crazy spread, going from 2533 to 2648. The second set of groups the spread and STD deviation narrowed down but its still 33+ fps higher.

Is this normal or to be expected when going from virgin brass to used brass? Should I do anything different in my reloading process aside from not crimping? Could it be a neck tension issue possibly from inconsistent crimps? Any input and ideas will be greatly appreciated.

I still have another 55 rounds of the once-fired brass to test, we just ran out of time that day.

Thank you!

 

[TaperPin]

A really smart guy, probably an engineer, explained the increase in velocity as equaling the amount of energy required to stretch the brass to fit the chamber. Once it’s fire formed, that energy gets put into a little extra velocity.

 

[Koda_]

Ive never measued any difference in POI or velocity between virgin and fireformed brass.
But i would guess not annealing and trimming could affect it.

 

[Harvey_NW]

What kind of chronograph were you using? I've seen POI shift wildly from bayonet style like Magnetospeed.

Velocity/pressure increase on 1x fired with proper shoulder bump vs new, yes. POI shift, not that I've ever noticed from that alone. It's possible you're seeing some small sample noise. I would also suspect the velocity variability could be from inconsistent crimp, as you mentioned.

I would load up 13-15 for test/zero with no crimp, and shoot a 10 shot group. Adjust zero, confirm with the remaining 3-5, and roll on.

 

[AM_Hunter]

What kind of chronograph were you using? I've seen POI shift wildly from bayonet style like Magnetospeed.

Velocity/pressure increase on 1x fired with proper shoulder bump vs new, yes. POI shift, not that I've ever noticed from that alone. It's possible you're seeing some small sample noise. I would also suspect the velocity variability could be from inconsistent crimp, as you mentioned.

I would load up 13-15 for test/zero with no crimp, and shoot a 10 shot group. Adjust zero, confirm with the remaining 3-5, and roll on.

I was using a Garmin Xero C1 for the chronoing. I agree it could be small sample size, since I still have plenty of those rounds loaded, I will do another comparison with 10 of those and I will load up the remaining once-fired brass I have with no crimp, and a case trim as well, and see how the two compare with a 10-shot group.

 

[AM_Hunter]

So now im at a loss. I loaded up 10 rounds of once fired brass same way as I only do except this time I did not crimp the rounds and I did trim them to 2.005" and then chamfer and deburred. Went to the range to shoot them and only 4/10 went off, the other 6 did not go off. Primers look like they got smashed in but somehow did not fire. Same primers ive always used, from a box that had other fire no issues. Thought it may have been a rifle issue but went back to my other lot that I had not trimmed and did crimp and fired 15 of those no issues. Anyone have any ideas? Of the four that did fire they were almost 100 fps slower than the 1-fired that was cirmped and around 50-60 fps slower than the Virgin brass.

See below for the difference between the oens that fired and the ones that did not.

 

[Harvey_NW]

When you set your die up for FL resize are you doing it per the MFG instructions to "cam over" against the shellholder, or are you using a comparator with the correct insert to measure ~.002" shoulder bump?

The only thing I can think of is if you're not measuring shoulder bump, you may have excessive headspace from sizing the brass down too much. The cases that fired had a longer neck, so the mouth may have been contacting stopping the case from moving forward and allowing the firing pin to strike with enough force to ignite the primers. On those trimmed, the firing pin might be pushing the case forward in the chamber inducing a light primer strike. The non-ignition and low velocity seems to indicate that's a possibility.

 

[jfk69]

So now im at a loss. I loaded up 10 rounds of once fired brass same way as I only do except this time I did not crimp the rounds and I did trim them to 2.005" and then chamfer and deburred. Went to the range to shoot them and only 4/10 went off, the other 6 did not go off. Primers look like they got smashed in but somehow did not fire. Same primers ive always used, from a box that had other fire no issues. Thought it may have been a rifle issue but went back to my other lot that I had not trimmed and did crimp and fired 15 of those no issues. Anyone have any ideas? Of the four that did fire they were almost 100 fps slower than the 1-fired that was cirmped and around 50-60 fps slower than the Virgin brass.

See below for the difference between the oens that fired and the ones that did not.

View attachment 856240

View attachment 856239

If the top photo are the one’s that fired, they look pretty flat to me (pressure?). If the bottom one’s are the rounds that didn’t fire, those primers look pretty proud to me (sitting above the base). If that were the case they could fail to fire due to not being fully seated. What is your priming method?

 

[TaperPin]

Wow, that would be a weird thing to have happen. Lol

Sounds like bad primers and just dumb luck they happened on that load. You’ll know by the time the rest of that lot are shot.

Poor ignition definitely messes with velocity. I’ve had a shot impact 2 MOA away from where it should after it required recooking a couple times before it when off. In that case it was ice in the bolt.

The primer strikes look deep enough, but I also wouldn’t rule out a slow firing pin, especially if you were shooting in the cold. Old grease, debris in the bolt, or an old tired firing pin spring can do that.

 

[AM_Hunter]

When you set your die up for FL resize are you doing it per the MFG instructions to "cam over" against the shellholder, or are you using a comparator with the correct insert to measure ~.002" shoulder bump?

The only thing I can think of is if you're not measuring shoulder bump, you may have excessive headspace from sizing the brass down too much. The cases that fired had a longer neck, so the mouth may have been contacting stopping the case from moving forward and allowing the firing pin to strike with enough force to ignite the primers. On those trimmed, the firing pin might be pushing the case forward in the chamber inducing a light primer strike. The non-ignition and low velocity seems to indicate that's a possibility.

I currently have an older RCBS FL sizing die (which I believe is the non adjustable one) and im resizing based on RCBS directions of putting the ram all the way up and screwing in the die until it makes contact with the shell holder, then backing off the ram and turning the die another quarter turn.

Im also suspect on the resizing being the culprit of my problems. Im not entirely sure how to properly bump my shoulder back .002" so ive just been using the FL die to resize to SAMMI.

 

[AM_Hunter]

If the top photo are the one’s that fired, they look pretty flat to me (pressure?). If the bottom one’s are the rounds that didn’t fire, those primers look pretty proud to me (sitting above the base). If that were the case they could fail to fire due to not being fully seated. What is your priming method?

Top ones are fired and bottom ones are not fired (misfire). Do the top ones look like they are overpressured? I use a RCBS hand priming tool, its been consistent with the previous couple hundred rounds ive primed with it so id be surprised if it was the issue but its entirely possible. I usually visually check the primer afterwards to make sure its seated deep enough and not protruding from the bottom of the case.

Is there a tool I could use to make sure the primers are seated properly? Ive got a case gauge coming to make sure cases are good going forward, im gonna pull the bullets on the non-fired brass and test them with that as well.

 

[AM_Hunter]

Wow, that would be a weird thing to have happen. Lol

Sounds like bad primers and just dumb luck they happened on that load. You’ll know by the time the rest of that lot are shot.

Poor ignition definitely messes with velocity. I’ve had a shot impact 2 MOA away from where it should after it required recooking a couple times before it when off. In that case it was ice in the bolt.

The primer strikes look deep enough, but I also wouldn’t rule out a slow firing pin, especially if you were shooting in the cold. Old grease, debris in the bolt, or an old tired firing pin spring can do that.

Rifle only has a couple hundred rounds through it, im gonna do a good cleaning of it but thats the first rounds that its ever happened to. Also its around 65-70F here in SoCal. Weirdly enough the rounds like did fire land almost spot on to my zero from my virgin brass while the once fired brass thats been having higher and less consistent velocities is around 1 moa right.

 

[SDHNTR]

I currently have an older RCBS FL sizing die (which I believe is the non adjustable one) and im resizing based on RCBS directions of putting the ram all the way up and screwing in the die until it makes contact with the shell holder, then backing off the ram and turning the die another quarter turn.

Im also suspect on the resizing being the culprit of my problems. Im not entirely sure how to properly bump my shoulder back .002" so ive just been using the FL die to resize to SAMMI.

This is your problem. You bumped shoulders back too far, increased headspace and got light primer strikes on the misfires. On the ones that did fire, that extra space made the case slam back against the bolt face and flatten. Get a comparator kit.

Stop doing so much different stuff and refine your process to be consistent. Measure, and do everything the same way each time.