Does bumping the shoulder back too far result in a notable velocity change?

[Goodcat]

So I've always bumped the shoulder back 2 thou after fire forming my whole life. Now, here I am sitting on 100 pieces (fire formed) of lapua straight 284, accidentally shoulder bumped 8 thou. I don't want to talk about it...

There is a SLIGHT difference in case capacity, but from your experience, will this result in any appreciable velocity difference next time, or just higher SD? To say, if I tune the load using fire formed brass that is bumped 8 thou, will the tune results be the same as when I bump it 2 thou after this firing?

Was planning on fine tuning the node and doing seating depth tomorrow, then realized my error.

Edit: Did just realize that seating depth is in relation to shoulder bump, so I’ll hold off until next firing on that

UPDATE: They all fired fine. Fine tuned the node and seating depth, and I’ll load 4 thousands longer after bumping 2 thou this time around to equate the same jump to lands. SD of 6.9 over 33 rounds after fine tuning. I’ll take that.

 

[Eagle Six]

Awh, come on let's talk about, you will feel better once you get it out in the open! That's what my dad often said. Thing is, it never really felt that good to spill the bean on my stupid tricks!!

 

[FeMan]

The only appreciable change I would expect from that is reduced case life from incipient head seperation later on. I cant see it afecting tune, especially if you are going to fine-tune seating depth with it as a second stpe with normal sized brass.

 

[moajoe]

My experience with a 6 dasher is that if the shoulder is pushed too far back the round may not go off at all. So to answer your question, yes, the velocity was 0.

 

[Bugs]

I would load a long bullet, a really hard jam with a lot of neck tension to be sure to hold the case back against the bolt face. The cases should re-fireform back OK.

 

[SKWERLZ]

I would load a long bullet, a really hard jam with a lot of neck tension to be sure to hold the case back against the bolt face. The cases should re-fireform back OK.

This!

 

[Codeman]

Along with loading you bullets long, you could also lightly oil the cases so they won't grip the chamber walls and stretch the case down at the base.

 

[dmoran]

Along with loading you bullets long, you could also lightly oil the cases so they won't grip the chamber walls and stretch the case down at the base.

Oiling the cases can create pressure spikes and can be very RISKY .......
Advising to do it lightly may be one thing, but not advising of the risk factors in doing so, if not done correctly and ever so sparingly is risky advise. Especially without further advise or suggesting to simultaneously mop out of the chamber between each fired round.

Hope someone doesn't get hurt, from reading such undetailed advise, with no warnings.
Personally HIGHLY ADVISE NOT TO ..... EVER !.!.!
( as I believe the @Forum Boss may as well )

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[Goodcat]

Oiling the cases can create pressure spikes and can be very RISKY .......
Advising to do it lightly may be one thing, but not advising of the risk factors in doing so, if not done correctly and ever so sparingly is risky advise. Especially without further advise or suggesting to simultaneously mop out of the chamber between each fired round.

Hope someone doesn't get hurt, from reading such undetailed advise, with no warnings.
Personally HIGHLY ADVISE NOT TO ..... EVER !.!.!
( as I believe the @Forum Boss may as well )

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Ya, no way I’m lubing a round. Oof.

Update: They fire formed just fine. The final measurement after checking from fire formed was a 6 thou bump, so I found my seating depth node and will seat 4 thou longer tonight with proper shoulder bump.

 

[dellet]

Trying to understand the reason for seating the bullet deeper, to compensate for the shoulder moving forward?

Seems like between changing the jump, and load density, it’s a much bigger change to the jump.

 

[Goodcat]

Trying to understand the reason for seating the bullet deeper, to compensate for the shoulder moving forward?

Seems like between changing the jump, and load density, it’s a much bigger change to the jump.

When the primer is struck, the case moves forward before the bullet exits. So currently at 6 thou bump, once I’m at 2 thou, I’ll need the bullet seated 4 thou deeper in the case, so when the case is driven forward before the bullet exits, the bullet depth to lands relationship will be the same as during my testing with the 6 thou shoulder bump. Essentially, the bullet is jumping 6 thou forward before leaving the case now. Because it’ll be only jumping 2 thou forward with my new brass, seating the bullet 4 thou deeper into the case will have the same o-give to lands relationship. Or am I thinking backwards? Lol

EDIT:
Yup, I think I am looking at this backwards. I’ll have 4 thou less into the lands from the shoulder difference, need to go 4 thou longer. I just edited my previous 2 posts so there’s no bad intel.

 

[dellet]

At least for me, that explanation in the video is very awkward.

If your case is 2” long, and you seat the bullet .250 deep, you can set the shoulder any where you want, and the “seated depth” will always be .250”.

What changes is the distance the case moves forward when the primer hits the case before either the shoulder contacts the chamber, or the bullet hits the lands.

There are two different things going on.

If jump is the distance the bullet travels from when the case shoulder is against the chamber, to the lands.

Then the farther the shoulder is set back, closer to the case head, the longer the neck becomes and the closer to the lands the bullet gets and jump decreases. The bullet needs to be deeper to maintain that distance.

The reverse is true, as the shoulder moves forward, shortening the neck, essentially pulling the bullet away from the lands. To maintain the same jump, the bullet would need to move forward, less deep.

 

[Goodcat]

At least for me, that explanation in the video is very awkward.

If your case is 2” long, and you seat the bullet .250 deep, you can set the shoulder any where you want, and the “seated depth” will always be .250”.

What changes is the distance the case moves forward when the primer hits the case before either the shoulder contacts the chamber, or the bullet hits the lands.

There are two different things going on.

If jump is the distance the bullet travels from when the case shoulder is against the chamber, to the lands.

Then the farther the shoulder is set back, closer to the case head, the longer the neck becomes and the closer to the lands the bullet gets and jump decreases. The bullet needs to be deeper to maintain that distance.

The reverse is true, as the shoulder moves forward, shortening the neck, essentially pulling the bullet away from the lands. To maintain the same jump, the bullet would need to move forward, less deep.

I appreciate that explanation very much. I’m glad you said something and that I rethought. So I’ll move 4 thou longer!

 

[searcher]

My experience with a 6 dasher is that if the shoulder is pushed too far back the round may not go off at all. So to answer your question, yes, the velocity was 0.

I'd say that holds true for any cartridge head-spacing off the shoulder. But .008" isn't going to create a misfire unless the case was not fully formed when the measurement for setback was taken. In my opinion - and contrary to what a few others voiced - I do believe that extra .006" setback can and likely will affect your tune (over his desired .002"). It won't necessarily destroy it - but is will not be as constant of a tune.

 

[Old Navy]

I guess I am just a little dense. I bump my shoulder .003". I was under the impression than when I close the bolt the spring loaded ejector is pushing the case forward. Am I missing something?

 

[Bill K]

I think you will find the extractor is holding the case, or should be, against the bolt face, so basically you are fireforming the shoulder back out to the chamber dimensions.

I guess I am just a little dense. I bump my shoulder .003". I was under the impression than when I close the bolt the spring loaded ejector is pushing the case forward. Am I missing something?

 

[pirate ammo]

Where's Guffy?

 

[perazziguy]

Where's Guffy?

Proclaiming to the masses (probably on TikTok), that he is the only one in the all creation that knows what happens to a cartridge inside a chamber...

 

[William Vogel]

My understanding is that when the firing pin hits the primer the case is shoved into the shoulder. When fire forming a short case, it’s been advised to lower the charge maybe a grain of powder and to set the bullet into jam in the lands to make sure the case head has the most opportunity to remain as far rearward during ignition and expansion of the brass. Without limiting the forward movement, the brass will stretch at the body towards the case head and thin in an area that is less than optimal. Bumping the shoulder back more than .002 will cause body stretch and weaken the brass. Sizing it after firing back short is going to continue to stretch the body. I’m not an expert on this but the logic makes sense to me.