Shoulder Bump

[rybe390]

Hey folks.

Long story short, I'm playing with shoulder bump measurements as I plan to prep over 1,000 pieces of 6.5 creedmoor brass. Using a FL sizing die.

Factory Hornady and Berger(Lapua) brass/loaded ammo all measures the same for me using a headspace gauge, with incredible consistency. They both grow about .004" after one firing, again with consistency.

I know the rule of thumb is to bump .002" for bolt actions, and .004" for a semi auto, using a fired case to take that mesurement.

Am I being irrational in wanting reliable ammo that I can run in any rifle...and wanting to bump the shoulder .004" to .005", to return to where the factory brass lives? To me, factory brass spec seems like a good idea.

Besides brass life(and how much of an impact is this extra bump), is there any downside to loading my ammo back to factory spec, rather than the minimal required bump to get it to chamber? Are there any benefits to me going the factory spec route?

It's also occurring to me that a die, fully seated/squishing the shell holder, would still be within SAAMI, correct? It'd just be the maximum bump the die can achieve. Anything where you "back it out" is what we are discussing and fine tuning.

Using a FL Hornady die with expander ball removed. Using expander mandrel in another die.

Thanks!

 

[Dthomas3523]

Bump shoulder to the point a bolt without ejector and firing pin falls free with gravity.

Anymore than that is unnecessary.

 

[308pirate]

Am I being irrational in wanting reliable ammo that I can run in any rifle...and wanting to bump the shoulder .004" to .005", to return to where the factory brass lives? To me, factory brass spec seems like a good idea.

You can certainly do that. Just expect a lower number of reloads out of your brass before you start seeing issues like thinning (and eventually splits) just above the case head.

"Factory" specs are not always the best. It all depends on what you need to do.

 

[MtnCreek]

Don't assume a die setup to contact the shell holder will size within spec.

 

[TheOfficeT-Rex]

There's no additional downside to bumping 4-5k, except for brass life, and you may have to trim more often as the brass flows from the sizing process.

I'd also make measurements along the base diameter to make sure your sizing die adequately sizes the base, if you want to ensure it will run in anything. Or you could use a case gauge, but who buys those for rifle calibers anymore?

 

[Steel head]

I generally bump it back minimally, about 1 thou.
For a important situation I might increase that a tad for extra insurance.

 

[kingaling]

Don't assume a die setup to contact the shell holder will size within spec.

I learned this the hard way... those directions that come with the die that tell you to screw in the die until it touches the shell holder then go 1/4 turn deeper... Not exactly the best idea ever.

 

[Matches Malone]

I had no idea that bumping back 2 thou was a rule of thumb. I always used it as a starting point when trying to get the bolt to freely fall (with no ejector of firing pin). Sometimes that is all it would take, sometimes it would be more or less depending on the barrel and brass in question.

 

[Haney]

I had no idea that bumping back 2 thou was a rule of thumb. I always used it as a starting point when trying to get the bolt to freely fall (with no ejector of firing pin). Sometimes that is all it would take, sometimes it would be more or less depending on the barrel and brass in question.

Same here , too many variables to use 2 thou bump as a rule .

 

[Dthomas3523]

Most every heavy/hard bolt lift/extraction issue I’ve seen, they shooter always says “I bumped them back .002.”

Remember we are using comparators and other such things. So we aren’t actually getting perfect measurements. Not to mention different chambers from rifle to rifle.

Instead of referencing the amount the shoulder is pushed back, test and see what measurement from base to shoulder functions properly in your rifle. Then just make sure you’re within .001-.002 of that measurement.

It also helps to make sure that measurement allows a bolt without ejector/firing pin to fall completely free with gravity. That way if your bump is a thou or so too long, it will still chamber with a slight amount of resistance and also still extract properly.

 

[eschafer]

fire a round, take the case home and reload it sizing neck only... fire, reload (neck size only) and fire it again.... now mark that case and put it away safe so you can use it with your comparator each time you decide to reload again, for that rifle... it's forever holding your base to shoulder measurement of your specific chamber... Now, with a FL die or Body Die, bump your reloads .001"-.002" shorter than that case/gauge...(check sized brass for bolt resistance and extraction)..... call it your "chamber gauge"..... you might only need to fire it twice, but you get the idea..