The saga of shrinking brass...

[Outdoored]

OK, so I ordered 100 new brass casings and measured them all....The majority wound up at 2.695. So....I loaded some of them up to test. The loads:

Win WSR primer
FTX 225g
Cases were loaded just like I got them...no chamfering, no debur, no flash hole reaming, nothing....
Lil gun powder starting load 37.5g, then 38.0, then 38.5, then 39.0
Each loading was 5 rounds.

Accuracy was PHENOMINAL on the 38.5g and 39.0g....under an inch at 100 yds with 38.5 and opened up to 1.5" with the 39.0
5 shot avg velocity at 38.5g was 2265, 39g velocity avg was 2286
My question:
Measuring the brass after the first firing.... they shrunk.....some cases now are as short as 2.683, but ALL cases shrunk a good bit and it appears the lower charge shrunk the most while the upper charges shrunk the least....what gives??? Glad I didn't trim!!

Also...did I make a mistake by loading the brass as-is from the factory? Should I have done more?

[Outdoored]

Sorry...forgot to include the gun: Bushmaster 20" barrel, RR arms lower with NM trigger. Temp was 88*

[Hoot]

Ed, what you experienced is par for the course. Nothing you did wrong. It just happens.

When I first started shooting and reloading, it took me a while to come to the same conclusion as you observed. That the more stout the charge, the less the shrinkage. I suspect It is a function of how tightly the case grips the chamber walls during ignition and rise in pressure. Too little pressure (grip), the more the forward half of the case shrinks back when it expands. Equally important is how quickly it comes up to pressure. If it comes up too slowly, the rearward thrust of the case starts it moving rearward before it finally grips the chamber. That can be seen when the case head imprints from the bolt extractor and ejector.There is a window for this, where after some point, the entire case grips the chamber wall.

There will be shrinkage, regardless of what you do. It is unavoidable. The good news is that the shrinkage is the worst after the first firing. The amount of shrinkage in subsequent shootings is less each time and eventually stops. The shrinkage will not impact performance adversely, but I choose to sort my cases by length as well as the number of times it has been reloaded. Here's a good way to keep track of how many times the brass has been shot. By sorting your brass by length, you can minimize the variation in the amount of tension imparted by the taper crimp. The die is in a fixed position and it needs to engage the end of every case the same amount. If they're different lengths, that dies will impart different amounts of taper. I sort mine in .005 increments. IE 1.695, 1.690, 1.685, 1.680, etc. If you choose to crimp your loads utilizing a modified Lee Factory Crimp Die that imparts a stab crimp to the sides of the case, below the mouth, then shrinkage sorting is not necessary. Here's a step by step process for modifying a Lee FCD, complete with pictures.

Hoot

[Outdoored]

I knew you'd come along and give me an answer!!
I think I've decided on staying with the 225...it's cheap, and my most accurate load so far. My only concern is now I need to see if once and twice fired brass shoots the same, as these were new cases, and seemed to have more neck tension while seating (I don't use an expander die). There wasn't any sooting in any of these loads. I forgot to add that I taper crimped to .475.

My 240g XTP mag experiment is frustrating.....the only decent group I get comes from 34g lil gun...everything else just opens up to a horrible mess.

[wildcatter]

OK, so I ordered 100 new brass casings and measured them all....The majority wound up at 2.695. So....I loaded some of them up to test. The loads:

Win WSR primer
FTX 225g
Cases were loaded just like I got them...no chamfering, no debur, no flash hole reaming, nothing....
Lil gun powder starting load 37.5g, then 38.0, then 38.5, then 39.0
Each loading was 5 rounds.

Accuracy was PHENOMINAL on the 38.5g and 39.0g....under an inch at 100 yds with 38.5 and opened up to 1.5" with the 39.0
5 shot avg velocity at 38.5g was 2265, 39g velocity avg was 2286
My question:
Measuring the brass after the first firing.... they shrunk.....some cases now are as short as 2.683, but ALL cases shrunk a good bit and it appears the lower charge shrunk the most while the upper charges shrunk the least....what gives??? Glad I didn't trim!!

Also...did I make a mistake by loading the brass as-is from the factory? Should I have done more?

IMO, I don't think your measurement AFTER the firing is of any importance or can ever really give useful Data. Hoot has it right about the upper end loads gripping the chamber better, and shrinking less, but again, this data is not useful to me until after you FL-resize the once fired case.

My concern would be after you fire the cases and FL-resize, "Now"what are the lengths?

Use a sharpie and mark the case up with data, such as length before you shoot and then measure, obtaining a before and after measurement, and never do this to virgin brass. Try doing the suggested to your once fired cases. I usually do not get accurate data until the cases have fired 2 or 3 times.

Hoots push punch is a real tip, one I use often since he recommended the idea..

..t