Case Trimming when Headspace Reloading

[CTigers19]

Hello,

I wanted to try my first attempt at “bumping the shoulder” back ~0.002” when resizing fired brass rather than using the standard die instructions to size for the minimum chamber spec.

This would typically result in a longer case bottom-shoulder dimension than factory ammo by not pushing the shoulder back quite as far.

Those of you that have done this, how do you handle trimming cases? If I were to trim to the case trim length on the print, I would have a shorter neck since the shoulder is out farther. Are you guys keeping your brass just under max length and calling that good?

 

[Maurygold]

Measure 15 after bumping and trim to the shortest

 

[CTigers19]

Measure 15 after bumping and trim to the shortest

Thanks, I”ll give that a shot. If that value were to exceed the printed max case length, would I potentially be risking contacting the case mouth in the chamber before the shoulder hits?

 

[Schütze]

Thanks, I”ll give that a shot. If that value were to exceed the printed max case length, would I potentially be risking contacting the case mouth in the chamber before the shoulder hits?

Everything is guessing until you have your reamer specs or even better measurements of YOUR chamber.

 

[KnowNothing256]

Thanks, I”ll give that a shot. If that value were to exceed the printed max case length, would I potentially be risking contacting the case mouth in the chamber before the shoulder hits?

Almost certainly not, most chambers have more than enough room for case neck length.

The best method is to chamber an empty case and run a borescope into the muzzle. Then you can see exactly where the neck is lining up in the chamber.

I think you’ll find you have tons of room.

 

[Schütze]

 

[CTigers19]

Everything is guessing until you have your reamer specs or even better measurements of YOUR chamber.

Good call. I have a MPA rifle so they include some chamber measurements. Always thought of that as getting a new barrel cut but would be relevant here. I’ll be home in about an hour and post it if you can give me some insight on what to do with it

 

[JerkyFreak]

Chambers are cut to some sort of a standard, usually SAAMI or CIP. Unless you have a true wildcat, neck length are usually to standard; differences are usually freebore length/diameter and/or throat angle. Even if you trim to SAAMI spec, you would not trim more than 10 thou.

So, don't wory about neck length, provided they are all the same.

 

[ma smith]

As people have mentioned, there's a gap built into the reamer print to allow some clearance beyond the spec for neck lengtht. You can buy a tool from sinclair to measure this--ie your chamber's neck length/ clearance, its like $6-7 dollars (or something pretty reasonable).

The Sinclair Chamber Length Gauges are designed so the handloader can determine the true length of his rifle's chamber when measuring from the bolt face to the end of the chamber's neck. This measurement will tell you how much excess case length your chamber may have over published trim lengths in reloading handbooks.

SINCLAIR INTERNATIONAL Sinclair Chamber Length Gage

Check out SINCLAIR INTERNATIONAL Sinclair Chamber Length Gage available Online at Brownells Today and many more Case Length Gauges products are available in our Reloading Department.

www.brownells.com

 

[Rocketmandb]

The best method is to chamber an empty case and run a borescope into the muzzle. Then you can see exactly where the neck is lining up in the chamber.

Exactly this. I tend to trim once at first firing, then let cases grow to close to the end of the chamber, then set that as the trim length. This helps lower the amount of carbon deposited at the end of the chamber.

Leading up to that, I don't sweat small differences in neck length in the least.

Here's why:

Let's say your neck length is .300", and your spread on your OAL is .003" or 1%. The only impact neck length has on a shot is the friction it exhibits on the bullet. Friction is directly related to bearing surface area, meaning that if you've got 1% variance in length, you've got 1% variance in friction due to that length. Typically, I see ~35-60 lbs of seating force depending on the round. .35 - .6 lbs of difference in seating forces will have an impact, but it is a very small one considering that for a seating force averaging like 60lbs, best case you're getting a 10 lb spread in seating forces. That would change to a 10.6 lb spread

Just guessing here, but the change in SDs would be like .1 or .2 fps at most (which you can't practically measure). And this would only be for a few rounds until all the cases "catch up" in length to the max-length cases.

 

[CTigers19]

Everything is guessing until you have your reamer specs or even better measurements of YOUR chamber.

Maybe not what we are looking for. They gave:
Action face to cartridge seat: 0.823
Tenon length: 0.760
Head space dimension: 0.61
Manson reamer
pitch diameter: 1.06

 

[CTigers19]

Almost certainly not, most chambers have more than enough room for case neck length.

The best method is to chamber an empty case and run a borescope into the muzzle. Then you can see exactly where the neck is lining up in the chamber.

I think you’ll find you have tons of room.

Do you have a recommendation on a bore scope?

I’m also using hornady match dies for the first time so the neck sizing bushing is new to me. I followed their instructions by measuring a loaded round and subtracting 0.002/3” off but any advice on that would be great too

 

[KnowNothing256]

Most everyone uses a Teslong of one variety or another. They’re cheap and mostly get the job done.

Opinions vary on whether or not to own one, but I find mine useful.

 

[CTigers19]

Exactly this. I tend to trim once at first firing, then let cases grow to close to the end of the chamber, then set that as the trim length. This helps lower the amount of carbon deposited at the end of the chamber.

Leading up to that, I don't sweat small differences in neck length in the least.

Here's why:

Let's say your neck length is .300", and your spread on your OAL is .003" or 1%. The only impact neck length has on a shot is the friction it exhibits on the bullet. Friction is directly related to bearing surface area, meaning that if you've got 1% variance in length, you've got 1% variance in friction due to that length. Typically, I see ~35-60 lbs of seating force depending on the round. .35 - .6 lbs of difference in seating forces will have an impact, but it is a very small one considering that for a seating force averaging like 60lbs, best case you're getting a 10 lb spread in seating forces. That would change to a 10.6 lb spread

Just guessing here, but the change in SDs would be like .1 or .2 fps at most (which you can't practically measure). And this would only be for a few rounds until all the cases "catch up" in length to the max-length cases.

I’m using hornady match dies for the first time so the neck sizing bushing is new to me. I followed their instructions for sizing, but any advice there would be great. Do guys use any sort of crimp when using those ?

 

[Rocketmandb]

I’m using hornady match dies for the first time so the neck sizing bushing is new to me. I followed their instructions for sizing, but any advice there would be great. Do guys use any sort of crimp when using those ?

Well, don't feel too out of sorts. First off, the bushing simply impacts the outside diameter (OD) of the case when you size. Larger bushing size = larger OD, smaller bushing size = smaller OD.

Likely, the Hornady instructions are something similar to:

- Seat a bullet in a dummy case.
- Measure the OD of the case at a point where the bullet is definitely engaged with the neck and measure. This measurement gives you the width of the bullet + 2x the average of the thickness of the case wall at the measured points. For example, for a 6mm bullet (.243") if one side of the case mouth is .014" and the other is .012", the measurement would be .014" + .012" + .243" = .269"
- You want to choose a bushing that will yield something smaller than .269" because when you seat a bullet, you need some "neck tension" that applies a force from the brass to the bullet so the bullet can effectively be held in place.
- You can try different bushing sizes like maybe, in the example I gave, .267" or .266"
- After sizing using a bushing, instead of using an expander ball on your sizing die, I (and many others) like to run the case through a mandrel die that will slightly open up the neck a little after it's been sized down.
- I like to find a bushing size that sizes every piece of brass such that the mandrel engages every case, but just minimally. That means you're engaging the brass as little as possible throughout the entire process.

Mandrel Die:

https://21stcenturyinnovation.com/buy-online/ols/products/expander-die-body/v/903

Mandrel (choose for your caliber):

https://21stcenturyinnovation.com/buy-online/ols/products/black-nitride-expander-mandrel/v/NBE24

 

[CTigers19]

Well, don't feel too out of sorts. First off, the bushing simply impacts the outside diameter (OD) of the case when you size. Larger bushing size = larger OD, smaller bushing size = smaller OD.

Likely, the Hornady instructions are something similar to:

- Seat a bullet in a dummy case.
- Measure the OD of the case at a point where the bullet is definitely engaged with the neck and measure. This measurement gives you the width of the bullet + 2x the average of the thickness of the case wall at the measured points. For example, for a 6mm bullet (.243") if one side of the case mouth is .014" and the other is .012", the measurement would be .014" + .012" + .243" = .269"
- You want to choose a bushing that will yield something smaller than .269" because when you seat a bullet, you need some "neck tension" that applies a force from the brass to the bullet so the bullet can effectively be held in place.
- You can try different bushing sizes like maybe, in the example I gave, .267" or .266"
- After sizing using a bushing, instead of using an expander ball on your sizing die, I (and many others) like to run the case through a mandrel die that will slightly open up the neck a little after it's been sized down.
- I like to find a bushing size that sizes every piece of brass such that the mandrel engages every case, but just minimally. That means you're engaging the brass as little as possible throughout the entire process.

Mandrel Die:

https://21stcenturyinnovation.com/buy-online/ols/products/expander-die-body/v/903

Mandrel (choose for your caliber):

https://21stcenturyinnovation.com/buy-online/ols/products/black-nitride-expander-mandrel/v/NBE24

I’m shooting 7mm but yes, that bushing selection is pretty close to what they say. They also offer 10 sizes and doing that put me smack in the middle so that feels good (with no data to back that up)

So they ship their sizing die with a decapping pin retainer that does not contact the case mouth ID at all. They also give you al elliptical resizer that will set the case mouth to whatever that set OD of the tool is, but they say that can mess with the neck tension/bushing you select

So I was thinking if you don’t touch the inner case mouth, and use the bushing for the outer, with a pretty heavily boat tailed round, and a slight chamfer on the case mouth you could probably seat bullets no problem, but you won’t necessarily have a uniform wall thickness.

Using the elliptical expander, you’d set the ID and OD, but give the brass nowhere to go but make the neck longer needing to be trimmed and might have weird neck tension?

 

[Schütze]

As people have mentioned, there's a gap built into the reamer print to allow some clearance beyond the spec for neck lengtht. You can buy a tool from sinclair to measure this--ie your chamber's neck length/ clearance, its like $6-7 dollars (or something pretty reasonable).

SINCLAIR INTERNATIONAL Sinclair Chamber Length Gage

Check out SINCLAIR INTERNATIONAL Sinclair Chamber Length Gage available Online at Brownells Today and many more Case Length Gauges products are available in our Reloading Department.

www.brownells.com

They are often out of stock.

If you use a resized case, fire formed in your chamber and cut the neck in half like in the above picture you could obtain the exact measurement for trim length. Easy and quick.

If the neck tension is slightly higher on the case you can get the bullet touching the lands measurement as well at the same time.

I usually have at least one dummy rd for each bullet per caliber anyway. Makes it easier to troubleshoot an issue if there is one down the road for me.

Lot’s of guys seem to be fine with just notes, I prefer the above dummy rd as well, just for comparison of all case dimensions.

 

[Marine52]

They are often out of stock.

If you use a resized case, fire formed in your chamber and cut the neck in half like in the above picture you could obtain the exact measurement for trim length. Easy and quick.

If the neck tension is slightly higher on the case you can get the bullet touching the lands measurement as well at the same time.

I usually have at least one dummy rd for each bullet per caliber anyway. Makes it easier to troubleshoot an issue if there is one down the road for me.

Lot’s of guys seem to be fine with just notes, I prefer the above dummy rd as well, just for comparison of all case dimensions.

I like this method better than using the Sinclair Chamber Length Gauges. The Sinclair gauges will give you a false chamber length that exceeds the true length shown by your reamer print (bolt face to end of neck).