Forming IMP/AI Brass. . .Question.

[STEEL SLINGER]

When forming to an Improved or Ackely case dimension do you loose in overall case length, neck length . . .where? It just seems to me that somewhere after decreasing the case body taper and blowing the shoulders out to 40* that somewhere, something, would have to give. Reason for asking is I have a case (6.5x55 Swede) that I want to possibly go the IMP route with, but I don't really want to loose my case neck length. Anyone with IMP case forming knowledge please fill me in. Thank you.

 

[MudRunner2005]

You lose on average about 0.020" at the very end of the case mouth from the case having to scavenge more brass from somewhere to fill the walls and sharper shoulder angle of the improved chamber.

 

[STEEL SLINGER]

MudRunner - "" You lose on average about 0.020" at the very end of the case mouth from the case having to scavenge more brass from somewhere to fill the walls and sharper shoulder angle of the improved chamber. ""

Thank you for the reply;
So, it would be best to use a case that has a few firings (significant brass flow) on it that needs to be trimmed for your first fire formed case and work for your desired case neck length from there. Would it also be possible to run several strokes in a full length sizing die or some sort of specialty die to achieve the same neck stretch?

 

[JeffVN]

+1 the case gets shorter in overall length, with the neck shrinking (It's probably taking from other locations as well, but the one I can see and measure is the length of the neck.)

JeffVN

 

[Bill Johnson]

MudRunner - "" You lose on average about 0.020" at the very end of the case mouth from the case having to scavenge more brass from somewhere to fill the walls and sharper shoulder angle of the improved chamber. ""

Thank you for the reply;
So, it would be best to use a case that has a few firings (significant brass flow) on it that needs to be trimmed for your first fire formed case and work for your desired case neck length from there. Would it also be possible to run several strokes in a full length sizing die or some sort of specialty die to achieve the same neck stretch?

No. Your concern over neck shortenning is misplaced. Even at trim-to length, you have over 3 caliber's neck length! Anything over 1.5 caliber is gravy. If you are concerned about concentricity, press-to-die alignment and neck wall variation will affect it more than anything else.

 

[LONGSHOOTER]

Kenny Jarrett suggests you fire form new brass only.

 

[STEEL SLINGER]

""BILL - No. Your concern over neck shortenning is misplaced. Even at trim-to length, you have over 3 caliber's neck length! Anything over 1.5 caliber is gravy. If you are concerned about concentrically, press-to-die alignment and neck wall variation will affect it more than anything else.""
__________________
Good Shooting,
Bill

Sorry. . .but could you explain what "3 caliber's neck length" and "over 1.5 caliber is gravy" means, please? I'm not sure of this. Thank you.

 

[Bill Johnson]

""BILL - No. Your concern over neck shortenning is misplaced. Even at trim-to length, you have over 3 caliber's neck length! Anything over 1.5 caliber is gravy. If you are concerned about concentrically, press-to-die alignment and neck wall variation will affect it more than anything else.""
__________________
Good Shooting,
Bill

Sorry. . .but could you explain what "3 caliber's neck length" and "over 1.5 caliber is gravy" means, please? I'm not sure of this. Thank you.

My apologies.

#1, I screwed the pooch on my math. 55mm length minus 47.13mm base-to-neck length = 7.87mm, so you only have a little over one caliber (6.5mm or .264") in neck length best case.

When looking at neck length, as a rule of thumb, whatever your caliber is, in this case .264", you want that much in neck length minimum to properly hold the bullet securely (given normal neck pressure). This is just a rule of thumb that's best adhered to with any rifle that's feeding from a magazine. I can go less then that on single-shots.

I would still say that you should just shoot them to fire form the case. If my math is right (this time), you will end up under your trim-to length but no worries. Folks have been fire-forming that case to the improved version a long time. I don't lose 0.020 on my 280AI, more like 0.012 or so but that's just my experience.

There's not much else you can do because the 6.5 x 55 has a different size case head then the typical 8mm/7mm Mauser/30-06 Springfield. It's a little larger thus forming a false shoulder on a bigger case (which is done to reduce case stretching when forming wildcat or improved cartridges) is not an option.

Again, my apologies for the confusion I created. Usually my math is spot on.

 

[MudRunner2005]

Here's what Bill is saying in basic terms...

At minimum, your case's neck should be as long as the diameter of your bullet you have chosen.

If you plan on shooting a 6.5mm bullet, you will want a minimum case neck length of .264".

If you choose a 7mm, you will want .284" as a minimum case neck length.

.30 caliber, .308" case neck length.

.338 caliber, .338" neck length.

And the list goes on...

 

[STEEL SLINGER]

Got it ! ! ! Thank you gentlemen. So, my Lapua brass, new, has a .299 neck and what you're telling me is that at most I'll loose .020, which in that case still has me at .279 and over my minimum in my case neck by .015?

Thanks again for the lesson.