Can Case Resizing Affect Velocity?

[257Roberts]

Got two of my three long range match guns ready to go. Both Winchesters, a Palma and a Creedmoor running 1/3MOA with single digit velocity SDs. Also have an RPA Quadlock 6.5x284 I am still working with. New barrel, now with 120 rounds thru it.

Last fall, 54.5gr H1000, 150gr SMK, 0.010" jump gave 2925fps. Round count at the time was 60.
This past week, same specs, same component lots, and only 8°F warmer I get 3015fps.
+90fps, seems odd, but I do know that barrels speed up when they "break in" a bit.

Average velocity aside, my SD and ES are terrible, 21/66. Shoots 1/4-1/3MOA at 100yds, I have no where longer to test at nearby.
I plan to do a ladder test, I know 100yds isn't typically enough, but it worked for my Palma rifle.

During my 8 round velocity string with the MagnetoSpeed bayonet attached, I had 3 or 4 cartridges that chambered really hard.
Meaning they obviously didn't have quite enough shoulder bump, and i had to bump the bolt handle forward pretty significantly to get the bolt to close. Going back and checking my prepped brass, it looks like a select few are .001-.0015" long at the shoulder, and these close with a "sticky" bolt. I know I was fiddling with my bushing die, so maybe I didn't get them all sized properly.

I wish I had marked down which shots the bolts closed hard on during my string, but it leads me to a question:
Will a noticeable difference in sized cases result in a noticeable change in velocity?

Intuition would tell me yes. That a cartridgewith less room to expand, will have a different pressure release curve than another cartridge with room to move. I assume the tighter case will have higher pressure, thus resulting in a higher velocity. Or am I totally off base here?

I know a 21/66 isn't going to work well at 1000 yards, so I'd like to get this figured out without blowing too many rounds thru the barrel. What puzzles the heck out of me is I am using the exact same high quality components, bushing dies and process on the other two rifles and they are working out great. Not sure what I am doing wrong here if it isn't this case sizing dilemma ...

Any insight would be greatly appreciated.

 

[jepp2]

Intuition would tell me yes. That a cartridgewith less room to expand, will have a different pressure release curve than another cartridge with room to move. I assume the tighter case will have higher pressure, thus resulting in a higher velocity. Or am I totally off base here?

A longer case should have greater internal volume. Greater internal volume would give lower peak pressure and that should translate to lower velocity.

I typically size my brass to be 0.001" shorter than the case head to datum where I can just "feel" the bolt close on the brass. To encounter the resistance on bolt closing you did, I would expect the cases to be quite a bit more than 0.001 to 0.0015 long.

 

[Ggmac]

What was the humidity and barometer reading . Anyway , yes In some instances Ive had velocity increase with resizing . I usually attribute it to neck tension , smaller case volume, cold welding , different lot number of bullets , primers or powder . Or powder has either gained moisture weight or lost it . Just some possibles that Ive blamed .

 

[Ned Ludd]

The difference in effective case volume for two cases where the shoulder of one is bumped .001" or .002" shorter than the other is negligible in terms of velocity. The theoretical difference is obviously not zero. However, the effect on velocity of such a small change in internal capacity will typically be less than the velocity SD for a 5 or 10 round string. Remember, the volume of the case when expanded to fit the chamber represents the true "pressure cell" volume, and 50-60K psi pressure is more than enough to expand both cases to the same size, regardless of whether one started with a CBTS measurement .001" or so longer than the other.

I rather suspect that both the 90 fps increase in velocity and the increase in ES/SD were caused by some other factor than a couple thousandths difference in shoulder bump. The round count on the barrel is one possibility, as you noted. The slight increase in temperature would also have an effect, although it shouldn't be anywhere near that large for an 8 degree increase. Sometimes things happen over the winter that almost defy understanding. The brass sat there for several months after being prepped, as did the primers and powder. Perhaps it is possible to figure out the exact cause, but it's also possible you could burn up a lot of barrel life without ever identifying the cause. My suggestion would simply be to re-work the load with the idea that whatever change has actually occurred will be stable for some time. In other words, tune it in again and let the rifle tell you where it wants to shoot. The higher ES/SD could simply be the product of the load no longer being optimized in the same way it was at a velocity that was 90 fps slower. Re-tweaking it may well bring those numbers back down where they were previously.

 

[257Roberts]

Humidity last week was about 30%, last fall more like 60%.
I didn't mark down any atmospheric pressures.
Brass was prepped back in March, but not primed until just before powder.
I use Lapua brass and Redding bushing dies.
I haven't ever neck turned Lapua brass yet, maybe I need to consider that.

I had never had problems closing the bolt previously. And it was a significant "bump" required to get the bolt far enough forward to close the handle. What was odd, is that they all extract just the same, light bolt lifts and no issues. Very confusing, I am still pretty baffled by the whole thing.

Other than a long shoulder, I am not sure what else would cause the bolt to close this hard?

What is also interesting is just chamber checking other resized brass, yes I have found a couple with a slightly sticky bolt close, but nothing that required a bump forward, the only ones that required that were some of my loaded rounds. Could I have done something stupid with my bullet seating die that I am not thinking about?

I've never soft seated bullets, and at this length they shouldn't have been, but I don't think it would take near as much force.

In past readings, I've heard the 6.5x284 is "at home" in the 2950fps range. I am now curious if backing down a little bit will solve some of these issues. Only testing will tell I guess.

Thanks as always folks. This is my go to spot for advice.
I like to think I know what the heck I am doing, but I am always running into something new.

 

[K22]

Average velocity aside, my SD and ES are terrible, 21/66. Shoots 1/4-1/3MOA at 100yds, I have no where longer to test at nearby.
I plan to do a ladder test, I know 100yds isn't typically enough, but it worked for my Palma rifle.

If I'm reading this correctly, your concern seems to be about the "terrible" SD and ES yet your achieving 1/4 to 1/3 moa at 100 yards.

I don't know too many shooters that have enough shooting skill to distinguish between that fine of precision and attributing the difference to a load issue. Does it matter what the SD and ES is if your achieving consistent 1/4 - 1/3 moa groups?

Since I don't shoot anywhere near the distances you mentioned, i.e. 1000 yards maybe it does matter so I apologize in advance if the question adds no value because of my lack of experience shooting at that distance.

I do know that velocity is affected by air density, bore conditions, and consistency in neck tension. Velocity can also be affected by cartridges exposed to widely different temperatures.

Personally, I would be more concerned with the "really hard" chambering in those rounds you mentioned. During your next reloading session you may want to measure the shoulder set back of the entire group of cases to gather some data on the adequacy of your die setting. Perhaps it may need to be adjusted.

I have some rifle / case combos where a .002" shoulder set back is necessary so all cases will chamber adequately where as others work quite well with less set back, i.e. .001. As cases age due to repeated firings and sizing I sometimes have to make die adjustments to obtain sufficient set back. This is fairly easy for me since I use Skip Shims to adjust the amount of sizing, i.e. I just slip in a thinner shim.

The type of lube, consistency in application of lube and consistency in working the ram can affect sizing consistency. Also cases hardened over repeated firings and may not hardened uniformly affecting sizing consistency. I think this why a lot of guys anneal.

A Wilson case gauge can also be used to do a quick qualitative screen of a large group of cases. In my experience, cases near or above the top step of the gauge will experience hard chambering in most rifles, especially those with SAMMI dimensions.

 

[dellet]

If everything was the same as last time in the process, then it's possible the problem is in what you were processing.

If the brass was harder or had more springback in the cases that were hard to chamber, then most likely the necks also did not size correctly.

You might be able to confirm that by resizing the same cases and checking both shoulder position and inside neck diameter.

 

[INTJ]

I measure each case after sizing. There is easily .002" variation with many presses. It's not all the presses' fault as how we lube will have an effect. My Prazipress cut that variation in sizing down significantly, but I still measure every case.

Also, brass work hardening can change how much they will size. Annealing helps that.

So my recommendation is twofold. First, measure every case after sizing and make sure they all are the same within .001". Second, like @Ned Ludd said, rework the load.

 

[Old Navy]

I would start by measuring the loaded neck diameter of any properly sized case that the bolt was hard closing on.

 

[Capt. Oblivious]

Was the round count 60 rounds down the tube when your speed was 2925fps and now with 120 rounds down your seeing 3015fps ? That's about what id expect from a barrel speeding up around 90/110 rounds.