Shoulder Bump vs OAL

[Rockhurr]

Hi there shooters! TRUE or FALSE. I saw a YouTube video where a shooter was educating how shoulder bumping can affect AOL upon firing pin strike and I am scratching my head. Basically, the guy said that if you shoulder bump your case by a certain amount (say 0.002”) that when your firing pin strikes the primer, it will push your cartridge forward by 0.002” before the ignition actually happens, thereby, causing your AOL measurement to be off by that much and must be deducted from your loaded round. For example, using a Hornady COAL gauge with a modified case that is 0.002” bumped and you measured 2.315” base-to-ogive where it touches the lands, your actual loaded round measurement should be 2.313” instead because the firing pin will push your cartridge forward 0.002” before the combustion happens.

So I’m scratching my head because isn’t it true that (assume rem 700) once you close your bolt the extractor claw grabs on to the case rim (pulling the case back towards the bolt face), while the ejector plunger pushes the case base forward which actually sandwiches the cartridge in a firm position that will not move when the firing pin strikes the primer? Of course, the case will enlarge after ignition causing the case to expand rearward towards the bolt face, but that’s the aftermath. Please chime in.

 

[Supersubes]

First things first, who cares, .002 means exactly dick. No, the extractor doesn’t hold the case against the bolt face. The ejector and firing pin blow will push the cartridge forward until its shoulder runs into the chamber shoulder. Most OAL measuring methods would account for this to begin with since the case would be in the forward position during the measurement, so its a moot point.

 

[ShtrRdy]

I would suppose the extractor behavior could be different on different manufactures bolts. If you remove the ejector plunger then place the case against the bolt face you would be able to see if the case can be moved forward and back.

 

[Rockhurr]

First things first, who cares, .002 means exactly dick. No, the extractor doesn’t hold the case against the bolt face. The ejector and firing pin blow will push the cartridge forward until its shoulder runs into the chamber shoulder. Most OAL measuring methods would account for this to begin with since the case would be in the forward position during the measurement, so its a moot point.

Well, I didn’t say that the extractor pulls the case against the bolt face, but holds it back as the plunger is pushing on it. And, I don’t believe that the plunger will push the case forward until the shoulder touches the chamber. If that is true then a shoulder bumped case would feel the same as an umbumped case when chambering a round. And, if you take an empty case and put it in the bolt face until the rim is grabbed you can feel that there is no slack trying to pull the case forward or backward because it’s sandwiched between the extractor and plunger.

 

[Rockhurr]

That would make sense. If the bolt doesn’t have a plunger then you might be able to sense that the case can move forward and backward due to head clearance between the bolt face and case. I guess this guy didn’t make it clearer between different bolt designs. And, the only time when you would need to account for the bump would be when you use a fired case as your modified case for measuring your AOL but shoulder bumps your loaded rounds. Thanks!

 

[Supersubes]

Well, I didn’t say that the extractor pulls the case against the bolt face, but holds it back as the plunger is pushing on it. And, I don’t believe that the plunger will push the case forward until the shoulder touches the chamber. If that is true then a shoulder bumped case would feel the same as an umbumped case when chambering a round. And, if you take an empty case and put it in the bolt face until the rim is grabbed you can feel that there is no slack trying to pull the case forward or backward because it’s sandwiched between the extractor and plunger.

There is space under the extractor to account for tolerances of the bolt manufacturer and brass varieties. It certainly has .002 of clearance, and would push the case forward to the extractor or chamber shoulder, whichever comes first.

 

[Rockhurr]

There is space under the extractor to account for tolerances of the bolt manufacturer and brass varieties. It certainly has .002 of clearance, and would push the case forward to the extractor or chamber shoulder, whichever comes first.

Yes… the plunger will push the case forward until the rim touches the extractor or the case shoulder touches the chamber shoulder PRIOR to the firing pin strike and because of this there is no additional forward movement of the case when the firing pin strikes the primer…

 

[Rockhurr]

Does anyone know the max headspace difference between a fire formed brass and a shoulder bump before the case shoulder no longer contacts the chamber shoulder? In other words, if a shoulder is bumped too much, say 0.004”, will the case shoulder still contact the chamber shoulder when loaded?

 

[Supersubes]

Does anyone know the max headspace difference between a fire formed brass and a shoulder bump before the case shoulder no longer contacts the chamber shoulder? In other words, if a shoulder is bumped too much, say 0.004”, will the case shoulder still contact the chamber shoulder when loaded?

It will once the firing pin hits the primer, or in any plunger ejector type action.

 

[Supersubes]

Yes… the plunger will push the case forward until the rim touches the extractor or the case shoulder touches the chamber shoulder PRIOR to the firing pin strike and because of this there is no additional forward movement of the case when the firing pin strikes the primer…

Just seeing this. You're underestimating how much room there is under the extractor.

 

[Rockhurr]

It will once the firing pin hits the primer, or in any plunger ejector type action.

That’s what I initially thought too, but because the plunger will only push the case forward until the rim contacts the extractor right? So wouldn’t that limit how far forward the case moves?

 

[Rockhurr]

Just seeing this. You're underestimating how much room there is under the extractor.

I don’t know how much room there is between the tip of the plunger and the underside of the extractor but on my REM 700 bolt in order for me to align a brass with the bolt I need to apply a good amount of pressure on the case to push it against the plunger to get the case straight. That tells me the plunger tip is already at the extractor if not already bit further out pass it… but what do I know.

 

[Supersubes]

I don’t know how much room there is between the tip of the plunger and the underside of the extractor but on my REM 700 bolt in order for me to align a brass with the bolt I need to apply a good amount of pressure on the case to push it against the plunger to get the case straight. That tells me the plunger tip is already at the extractor if not already bit further out pass it… but what do I know.

That’s what I initially thought too, but because the plunger will only push the case forward until the rim contacts the extractor right? So wouldn’t that limit how far forward the case moves?

Yes, which is way more than .004 generally.

 

[4O6shootist]

Distance from lands changes all the time. Shoot a 6 creed 100 rounds, your jump has now grown 0.005 lol, a 28 nosler 100 rounds, 0.010 with N570, I've measured. It's a reference, jump, base to ogive, coal, waterver you want to call it, constantly changing.

 

[Rockhurr]

Distance from lands changes all the time. Shoot a 6 creed 100 rounds, your jump has now grown 0.005 lol, a 28 nosler 100 rounds, 0.010 with N570, I've measured. It's a reference, jump, base to ogive, coal, waterver you want to call it, constantly changing.

True but I think you missing the point.

 

[Rockhurr]

Yes, which is way more than .004 generally.

So where will it stop? .005” bump, .006” bump, or 0.020”? For sure the case shoulder isn’t going to touch the chamber shoulder if the case is bumped 0.100” . That’s my question initially.

 

[Supersubes]

So where will it stop? .005” bump, .006” bump, or 0.020”? For sure the case shoulder isn’t going to touch the chamber shoulder if the case is bumped 0.100” . That’s my question initially.

Like I said early in this thread, it depends. This is not anything worth devoting any time thinking about.

 

[FredHammer]

Even if you shoot "jam" there won't be any noticeable pressure increase. What I did see were some incredible groups at touch of the lands, and Jam. (Jam is where touching stops)

 

[Doc Holiday13]

True but I think you missing the point.

Possibly both of us are missing the point, but everything being discussed is highly subjective. Rimmed cases headspace off the rim. Rimless cases headspace off the shoulder.
Bolt plunger isn't part of the conversation because not all bolts have a plunger
your firing pin probably probably goes a full .015 out of the bolt when the spring is released. the case portion of a chamber has a certain amount of "slop" in it. I believe SAAMI specs a minimum size on all their dimensions and gives allowable tolerance based on those dims.
headspace is all a measurement from the bolt face to the should angle cut.
Now, if your case neck is too long, its highly possible that it will prevent you from chambering. Thats mostly a problem with rimmed cases but rimless still need trimming too

However you don't want to bump your shoulder by an excessive amount because you loose case capacity and .002 is considered a "standard middle ground" to where your case will fit in the chamber and you don't overwork your brass

 

[4O6shootist]

If a guys bumping shoulder 002 from chamber fitment, the absolute most a cartridge can come ahead is 002. Now, choosing a seating depth, regardless of jump, touching, jammed, that has such a small window of tune, that a 002 change moves you out of accuracy, is a bad idea.

Find a tune window of at least 006-010 wide that has same precision and same poi. This usually occurs from 40-80k jump. The tune windows closer to the lands or jam are usually smaller 003-005 wide, although usually offer the smallest groups, just require more work tuning as throat wears. I don't suggest anyone try to load at touch, because that's a very fine line between a light jam, or small jump.

 

[918v]

There is more clearance between the case rim and the extractor hook than .002” or .004” or whatever. Spread your caliper jaws .002” to .004” and look how little room that is. Then consider brass variance, carbon fouling, brass shavings, and plain old dirt that have to be accounted for.

 

[918v]

So where will it stop? .005” bump, .006” bump, or 0.020”? For sure the case shoulder isn’t going to touch the chamber shoulder if the case is bumped 0.100” . That’s my question initially.

Think about it: with factory ammo you can have more than .010” of clearance between the case and breechface. If the extractor hook controlled headspace it would not be able to snap over the case rim with the ejector pushing forward on the case.

 

[Rockhurr]

Think about it: with factory ammo you can have more than .010” of clearance between the case and breechface. If the extractor hook controlled headspace it would not be able to snap over the case rim with the ejector pushing forward on the case.

The question was… how much shoulder bump can there be so that the shoulders no longer touches when chambered. I don’t think anyone’s answered the question.

 

[4O6shootist]

The question was… how much shoulder bump can there be so that the shoulders no longer touches when chambered. I don’t think anyone’s answered the question.

Thats easy, strip your bolt/ejector and size a case in 0.001 increments until your bolt handle(steel bolt knob preferred) will drop closed on its own with a case clipped in under the extractor. I feel light aluminum or Ti knobs don't have the mass to drop freely as soon the case is no longer touching chamber. Once the bolt drops freely on its own weight I'll usually size an additional 2 thou. This is what I call my shoulder datum for sizing. Fired/decapped cases usually measure 0.0025-0.003 longer than my shoulder bump.

 

[Rockhurr]

Thats easy, strip your bolt/ejector and size a case in 0.001 increments until your bolt handle(steel bolt knob preferred) will drop closed on its own with a case clipped in under the extractor. I feel light aluminum or Ti knobs don't have the mass to drop freely as soon the case is no longer touching chamber. Once the bolt drops freely on its own weight I'll usually size an additional 2 thou. This is what I call my shoulder datum for sizing. Fired/decapped cases usually measure 0.0025-0.003 longer than my shoulder bump.

Yes.. that would answer my question. So does your bolt drop freely with a .003” bump on a fired case?

 

[4O6shootist]

Yes.. that would answer my question. So does your bolt drop freely with a .003” bump on a fired case?

Yes it does, it almost drops freely on case bumped 001 from fired but there is the slightest resistance, that's what I'm using as basically my zero headspace dimension, then go 002 more than that.

 

[Rockhurr]

Yes it does, it almost drops freely on case bumped 001 from fired but there is the slightest resistance, that's what I'm using as basically my zero headspace dimension, then go 002 more than that.

Assuming that the bolt can still drop freely with a plunger ejector installed, do you think that the bolt will still drop freely with a 0.003” bump? Assuming the case is straight in the chamber..

 

[Supersubes]

The question was… how much shoulder bump can there be so that the shoulders no longer touches when chambered. I don’t think anyone’s answered the question.

Its been answered many times, you just dont like the answer. The bigger question is why do you care? Its completely inconsequential.

 

[Rockhurr]

Its been answered many times, you just dont like the answer. The bigger question is why do you care? Its completely inconsequential.

Why do you always try to keep your seating depth the same? Shoulder bumps matters.

Can you say that if one case has a shoulder bump of .002” and one has 0.004” and your OAL is 2.250” on both rounds, that your distance from bullet ogive to lands is still the same?

 

[Supersubes]

Why do you always try to keep your seating depth the same? Shoulder bumps matters.

Bump the shoulders to the same dimension, done.

 

[Rockhurr]

Bump the shoulders to the same dimension, done.

Never always consistent that’s why the question… sometimes you get 0.002” sometimes 0.003”., sometimes even 0.004”…

 

[Supersubes]

Never always consistent that’s why the question… sometimes you get 0.002” sometimes 0.003”., sometimes even 0.004”…

Sort out your sizing operation.

 

[Rockhurr]

Sort out your sizing operation.

That’s off topic.

 

[FredHammer]

The question was… how much shoulder bump can there be so that the shoulders no longer touches when chambered. I don’t think anyone’s answered the question.

Get lighter and smoke a case. Size until you don't see marks in the soot.

 

[Supersubes]

That’s off topic.

You are one dim bulb!

 

[4O6shootist]

Assuming that the bolt can still drop freely with a plunger ejector installed, do you think that the bolt will still drop freely with a 0.003” bump? Assuming the case is straight in the chamber..

Bolt will not fall freely with plunger ejector. It'll push the case up against the side of Chamber, won't be straight, don't do this. You need remove the firing pin assembly and ejector. I strongly dislike plunger ejectors, I mainly run zermatt and arc actions for this reason. Most plunger springs are under too much tension anyway, the few plunger ejectors I have, I trim coils off the spring until I can depress the plunger without much difficulty using my thumb nail. I don't have ejection issues either. Too much pressure and it'll dent all your case neck on the lug abutments on its way out the chamber, just unnecessary.

 

[Doc Holiday13]

Why do you always try to keep your seating depth the same? Shoulder bumps matters.

you don't bump your shoulder and seat a bullet in the same operation. shoulder bump of .002 gives sufficient clearance to chamber and not not over work the brass. so stop confusing shoulder bump with seating depth

seating depth is dependent on several factors and one of those factors changes slightly every few hundred rounds.

 

[Rockhurr]

You are one dim bulb!

You’re not very smart yourself..

 

[Rockhurr]

you don't bump your shoulder and seat a bullet in the same operation. shoulder bump of .002 gives sufficient clearance to chamber and not not over work the brass. so stop confusing shoulder bump with seating depth

seating depth is dependent on several factors and one of those factors changes slightly every few hundred rounds.

Not doing that at all.. but I assume you’re saying that a .002” and a .004” bump will still keep your ogive to lands distance the same so no need to adjust seating depth to accommodate the difference, right.

 

[918v]

The question was… how much shoulder bump can there be so that the shoulders no longer touches when chambered. I don’t think anyone’s answered the question.

That’s because every firearm manufacturer has a different idea of what the optimum clearance is.

 

[918v]

Never always consistent that’s why the question… sometimes you get 0.002” sometimes 0.003”., sometimes even 0.004”…

But that could be due to measurement error or to die setup or lube. All fixable.

 

[Chris1972]

Not doing that at all.. but I assume you’re saying that a .002” and a .004” bump will still keep your ogive to lands distance the same so no need to adjust seating depth to accommodate the difference, right.

Correct, the bullet is seated by the die to the base of the case. It’s literally called BTO. Sizing the brass any amount doesn’t effect that

 

[4O6shootist]

Correct, the bullet is seated by the die to the base of the case. It’s literally called BTO. Sizing the brass any amount doesn’t effect that

This guy's believes the amount you Bump shoulders is going to affect your seating depth to lands distance because the firing pin is going to push the case shoulder to the chamber prior to ignition. If you're bumping the proper 002-003k, that's all it'll over go forward, the primer is igniting as it's going forward, and then you have 60k psi pushing the case back to the bolt face. This whole question is stupid. If this possible case movement is happening and it's ruining your accuracy, pick more forgiving seating depth or jam your bullets into lands 10k, then the case CANT move. I feel half dumb for for even participating in this conversation. Out........

 

[Rockhurr]

This guy's believes the amount you Bump shoulders is going to affect your seating depth to lands distance because the firing pin is going to push the case shoulder to the chamber prior to ignition. If you're bumping the proper 002-003k, that's all it'll over go forward, the primer is igniting as it's going forward, and then you have 60k psi pushing the case back to the bolt face. This whole question is stupid. If this possible case movement is happening and it's ruining your accuracy, pick more forgiving seating depth or jam your bullets into lands 10k, then the case CANT move. I feel half dumb for for even participating in this conversation. Out........

I never said the firing pin will push the case forward… if anything I argue against that because I believe the case is held in place by the extractor hook in all plunger/extractor bolt configs. Erik Cortina says the case moves forward the amount of the bump… are you as good or better than Erik Cortina to argue the contrary? Don’t come in here telling people what you don’t know..

And if in case it’s too hard for you to understand, the seating depth will be the same regardless of shoulder bump, but when you chamber a round with a 0.002” bump versus one that has no bump, your bullet will not have the same ogive to lands distance… if you can’t picture this then go bang your head somewhere else.

 

[Chris1972]

Soooo…. “I THINK” you’re partially right, the case does expand to the shoulder of the chamber as well as the bolt face. Here come the I THINK part. The brass does not expand until the bullet contacts the lands and causes a pressure spike. Meaning the bullet moves before the shoulder making the shoulder bump irrelevant.

This is my theory, not necessarily a fact.

 

[Rockhurr]

Soooo…. “I THINK” you’re partially right, the case does expand to the shoulder of the chamber as well as the bolt face. Here come the I THINK part. The brass does not expand until the bullet contacts the lands and causes a pressure spike. Meaning the bullet moves before the shoulder making the shoulder bump irrelevant.

This is my theory, not necessarily a fact.

I’m not disagreeing with you but…

What I’m explaining here is all before firing… a lot of guys on here are saying that the case shoulder and chamber shoulder are always touching when chambered due to play or slop between the case rim and extractor claw to allow the plunger to push the case forward butting it’s shoulder against the chamber shoulder…that means that the shoulder can only be bump so much before the shoulder contacts no longer exist. So that’s where I ask the question how much bump is there before this happens…

 

[918v]

I never said the firing pin will push the case forward… if anything I argue against that because I believe the case is held in place by the extractor hook in all plunger/extractor bolt configs. Erik Cortina says the case moves forward the amount of the bump…

It does.

 

[Halffull]

I normally read a lot and post very little, but here's my thoughts.

1) It seems that that shoulder must move forward and touch the chamber in order for the firing pin to dent it far enough for ignition. This is proven with brass that is headspaced to short. Recently had some new Starline that was .010 shorter than fired brass in that chamber. I had a few that dented the primer and ftf. If the extractor provided a stop (headspace) for the cartridge then any headspacing would ignite.

2) The amount of bump certainly affects the distance to lands (jump). The BTO and COAL will be the same for two cases with different HS's because seating depth is indexed from the base of the case to the ogive. But if statement #1 above is correct and the case has to be forward against the chamber for ignition to occur, then the shorter HS case will yield a bullet that is closer to the lands. That is because the distance from the bullet ogive to the shoulder datum of the case (which is never really measured) will be longer by the amount of extra bump.

So, with good reloading practices and normal tolerances this shouldn't cause issues. I think it was stated above that .002 variation on a fair amount of jump doesn't amount to much. It would/could be significant if loading very near the lands.

 

[4O6shootist]

I never said the firing pin will push the case forward… if anything I argue against that because I believe the case is held in place by the extractor hook in all plunger/extractor bolt configs. Erik Cortina says the case moves forward the amount of the bump… are you as good or better than Erik Cortina to argue the contrary? Don’t come in here telling people what you don’t know..

And if in case it’s too hard for you to understand, the seating depth will be the same regardless of shoulder bump, but when you chamber a round with a 0.002” bump versus one that has no bump, your bullet will not have the same ogive to lands distance… if you can’t picture this then go bang your head somewhere else.

Once again, as I previously stated, if you're loading in such a tight tune window, that a variation of -002 seating depth blows your groups up, you're doing it wrong.
If you want to eliminate this variable from possibly happening, jam your bullets into lands at least 8k, less could still possibly move forward in the chamber 001 upon pin fall.
Cortina also says, don't chase the lands! But after shooting 100 rounds, he then retests a seating depth 005 further out, if it has the same precision, thats his new seating depth. What is he doing? CHASING the lands!
He also peddles his stripped to bare metal cleaning process with iosso and abrasives. This works for low volume shooting. It DOES NOT for anyone who's going to run more than 60-80 rounds between cleaning. You'll get a speed up ranging from 30-50 fps by the time you're around 150 rounds down the barrel. 50fps can really throw a load out of tune. Myself and two other local prs shooter bit hard on this cleaning process, it can be okay with a 1 day match, but you're up about 20-30fps. 2 day matches we were up 50fps and putting shots over targets, one guy getting psi signs. Went back to more traditional cleaning methods and this all went away. Iosso and abrasives still have their place, cleaning stubborn carbon rings, or a heavily copper fouled barrel. But should not be used aggressively every cleaning unless you plan to shoot no more than 70 rounds between barrel maintenance.

His processes are geared towards his benchrest/Fclass disciplines. These do not cross over directly to field tactical shooting, where a guy might for put 250 rounds down his rifle at a 2 day match. It works for him at his game. Doesn't mean its gonna work for your game or my game.

I shoot a lot, and I handload from 22 grendel to 338 rum with 16 others in between. I stand by my statement, if 002 change from bto to lands blows your accuracy or notably changes poi, you're doing it wrong.

 

[918v]

Just measured a case rim. The rim thickness on that one case varies .003” throughout it’s circumference.