Shoulder bumping question

[J.d. Popkes]

I'm sure you're correct especially if your experience indicates as such. Those in the ultra-precision game of benchrest and long-range precision shooting attempt to keep everything as consistent as possible and go to extra lengths to do so. I imagine, small variations can make a big difference in their game.

My needs are not as precise. Of course, I want the most accurate reloads I can produce but I balance that with trying to keep my reloading process as simple as possible. I have not noticed any POI changes (within my normal aggregates) shooting at 100 and 200 yards on paper or at varmints / predators up to 300 yards with bump variations of + or - .001, even an isolated occasionally .002 variation.

I’m not anyone special. But at 600+ yds I feel like in my testing it matters. I went through a phase where my bumps were varying a bunch. I solved that. Groups improved. I’ve got a long way to go, but this has been one of my eureka moments. But I had some variation of 5thou. Now I have zero thou.

 

[762willdo]

I can get to Zero myself but I do it by culling those that are “off” and by culling I mean they go in a box and are used for sighters etc. the interesting thing is I will use a few for sighters and the next time I bump them they fall back in line with the rest…..interesting….
if you have found a method of bumping where every case bumps exactly the same please share.

 

[JFrank]

Don’t lube the shoulders

 

[jccinohio]

Be interesting to see a test done.. take a load that drills bugholes and is say .001" headspace.. then shoot groups with .003" headspace, .005" headspace.. and see what happens

 

[762willdo]

Don’t lube the shoulders

I absolutely never lube the shoulders

 

[K22]

I’m not anyone special. But at 600+ yds I feel like in my testing it matters. I went through a phase where my bumps were varying a bunch. I solved that. Groups improved. I’ve got a long way to go, but this has been one of my eureka moments. But I had some variation of 5thou. Now I have zero thou.

I've never had variations of .005 but had some random .003's. I don't shoot those distances, but I certainly understand that variations are magnified at those distances. Basically, I strive for 1/2 to 5/8 moa at 100 yards as my qualifying load.

One of the policies I need to adopt is being careful sharing my experiences that might be helpful because all my experiences are 300 yards and under. This long-range shooting sport wasn't and isn't part of my experiences and I suspect requires more meticulous preparation of reloads. When I started in the late 60's, 200 yards was considered a decent shot for the average hunter, 300 yards was for special. Things have changed for sure but I'm still in the club.

 

[Bill K]

A lot of this should bumping is a matter of chasing your tail, round and round and not necessary. Size to your chamber, smooth and easy and just enjoy shooting.

 

[One Eye Pete]

I doubt that .0005 difference in shoulder bump alone would have any measurable effect on POI or accuracy.
On my target rifles I normally bump back until the bolt closes freely (consistently). This generally works out to be about .002. The main thing is to keep the shoulder bump as consistent as possible and also the bullet seating depth.
What I have definitely have seen make a difference in accuracy is a .003 change in bullet seating depth. The point here is that shoulder bump has an effect on actual bullet seating depth. When you pull the trigger the firing pin pushes the bullet forward towards the lands by the amount the shoulder is pushed back. An error in bumping the shoulder too much is the same as seating the bullet too long. Problems can arise when errors stack up. If the shoulder is pushed back by an extra .001 and the bullet is seated say .0015 long now you have a round where the seating depth is effectively off by .0025. This could have an effect.
When I load for a match I do everything possible to keep shoulder bump and seating depth consistent. Then I check each round with an Accuracy One seating depth comparator. This tool gives a shoulder to bullet ogive measurement which accounts for both shoulder bump and seating depth.
For a better explanation and info on the Accuracy One tool see the following article

Accuracy One Seating Depth Comparator Review « Daily Bulletin

Looking for a faster, easier, and more accurate way to measure bullet positions on your loaded rounds? Check out the Accuracy One Seating Depth Comparator.

bulletin.accurateshooter.com

 

[M-61]

So, I doubt a .0005 variation would have any affect, assuming you can measure to that level of precision

I agree 100%
5-10,000ths of an inch. Possible? Yes. Probable? No. Necessary? Not a chance.
Of course I’m talking about hand loading.

 

[J.d. Popkes]

I agree 100%
5-10,000ths of an inch. Possible? Yes. Probable? No. Necessary? Not a chance.
Of course I’m talking about hand loading.

Without reading the whole thread again did anyone say that a half thou might have a big impact?

 

[J.d. Popkes]

I doubt that .0005 difference in shoulder bump alone would have any measurable effect on POI or accuracy.
On my target rifles I normally bump back until the bolt closes freely (consistently). This generally works out to be about .002. The main thing is to keep the shoulder bump as consistent as possible and also the bullet seating depth.
What I have definitely have seen make a difference in accuracy is a .003 change in bullet seating depth. The point here is that shoulder bump has an effect on actual bullet seating depth. When you pull the trigger the firing pin pushes the bullet forward towards the lands by the amount the shoulder is pushed back. An error in bumping the shoulder too much is the same as seating the bullet too long. Problems can arise when errors stack up. If the shoulder is pushed back by an extra .001 and the bullet is seated say .0015 long now you have a round where the seating depth is effectively off by .0025. This could have an effect.
When I load for a match I do everything possible to keep shoulder bump and seating depth consistent. Then I check each round with an Accuracy One seating depth comparator. This tool gives a shoulder to bullet ogive measurement which accounts for both shoulder bump and seating depth.
For a better explanation and info on the Accuracy One tool see the following article

Accuracy One Seating Depth Comparator Review « Daily Bulletin

Looking for a faster, easier, and more accurate way to measure bullet positions on your loaded rounds? Check out the Accuracy One Seating Depth Comparator.

bulletin.accurateshooter.com

This is my point. All the steps in brass prep matter.

But I think ultimately, identical brass is what matters. If that makes more sense. It’s when I have random measurements of whatever step I’m measuring that I might see bs on my paper.

Primer depth
Bullet seat depth
Shoulder bumps
Explain how you can change one of these dimensions and not have an affect on the other two, or overall dimensions??

 

[6BRA]

Don’t lube the shoulders

And clean your die every 5 rounds, lube will migrate to die shoulders

 

[wildcatter]

.0005 is not worth worrying about, you'll never see that much change on the target.

 

[J.d. Popkes]

I’ll give it shot’
Primer depths ( I’ve tested at 500 yards with a click adjustable tool) have a window of no harm, set it to the bottom of the pocket with a slight crush and I have not seen any difference on paper.

Bullet seating depth changes of .001 will definitely show on paper at 500 yards.

Cases ..I’ve sorted by weight into 4/10 gr groups and tracked fps and single digit ES with a chronograph, shot those into a 500 yard charge ladder I can barely see a difference in point of impact from the charge increments so the sensitivity of case volume/capacity would need to be less than my margins to be definitive of the cause of shift, so .001-.003 headspace ..IDK my pea brain says I’m just not sure.

I don’t want to get into the weeds too deep because it’s not like I can prove to anybody on paper some formula. I too have a pea brain and what I’m saying is for example…….. shoulder bumps and seating depth are directly related. Primer seating depth variation combined with inconsistent headspace makes a difference. If it doesn’t then why can’t we, say, put a br round in a dasher and just fire form it? Why the false shoulder or seated way long? That might be an exaggeration of my theory.

I think it’s about consistency. Identical cartridges. I think if you can’t make identical cartridges you’ll get less identical groups. To me there’s a series of events going on when you fire a rifle and from the trigger pull and firing pin fall, all brass prep, powder and bullet variations, on and on all the way through recoil and follow through…. Matters.

And who knows how many other variables we can’t control. So I want to control everything that I can. And it takes exactly zero more time to make identical cartridges.

 

[6MMsteve]

Where is Guffey when we need him

 

[Murray Brook]

Where is Guffey when we need him

Last time I seen Guffey was about 6 weeks ago on Accurate Reloading.com and he was debating headspace.

 

[J.d. Popkes]

Where is Guffey when we need him

I’m assuming that’s directed at me?

 

[6MMsteve]

I’m assuming that’s directed at me?

no it's not directed at anybody think it's an ongoing joke, Guffey always debated shoulder bumpin, and we picked at him. I didn't have a clue about shoulder bumpin till 2015 or when I joined Accshooter..your good JD it's all good fun

 

[6MMsteve]

I’m assuming that’s directed at me?

you have probably forgot more than I know about reloading, whatever works is good by me

 

[6MMsteve]

I always try to headspace tight on the go and run about .002 to .003 shoulder bump and run about .010 off the lands if if I get lucky. It has worked for me my groups shrunk tons when I started or learned keeping the case tighter in the chamber, but I don't shoot comp anymore, and use off shelf dies.