Adjust bolt carrier return force?

[Bill Daniel]

DPMS LR 308 in 6.5 Creedmoor with Criterion 22” hybrid barrel, Superlative Arms adjustable gas block, and JP Enterprise Silent Captured recoil spring AR10 Gen. 2.
On releasing the bolt the cartridge COL will increase by 0.023” even with a reduction of neck size of 0.005” between prepped brass and loaded cartridge.
I have an e-mail into JP Enterprise this evening but don’t know if they can help.
Is modifying the recoil system what is needed or adjusting the gas block?
Still looking to improve accuracy by reducing cartridge trauma.
All the best,
Bill

 

[bacardisteve]

Thats an interesting problem. I would look into increasing your neck tension with some bushing dies.

 

[jetinteriorguy]

I know a lot of people don't like crimping bullets, and I have found at least in my gas guns I haven't needed it, but maybe a light crimp would help. The Lee collet crimp die works real well and is very easy to adjust for a very light crimp.

 

[Bill Daniel]

Adjust bolt carrier return force

Thanks fellows!
I used the bushing to increase the difference between the pre loaded case neck to the finished round by 0.005” with out benefit. I purchased some lighter springs from JP Enterprise and will try that next using the adjustable gas block to prevent bolt bounce.
I use the Lee collet crimp die for my hunting rounds in my Henry 45-70 and will purchase one for the 6.5 CM if reducing the return force is not effective.
All the best,
Bill

 

[stagpanther]

If I understand correctly--what you're saying is the inertial force of the BCG driven by the buffer spring is so forceful that its causing the bullet to
"creep" forward upon chambering? How are you verifying this? If you are manually ejecting the cartridge after dropping the BCG--are you sure that friction between the bullet and the chamber, throat, lugs etc might not be "pulling" the bullet out somewhat?

 

[44 AMP]

In addition to what Stagpanther asks, I'm ALSO wondering...

On releasing the bolt the cartridge COL will increase by 0.023”

increasing 0.023" from WHAT? exactly?

Am I understanding this correctly, that you chamber a loaded round, then eject it, and measure the COAL, and its 0.023 longer than it was when you loaded it tine the magazine???

IS the bullet NOW up against the rifling??

 

[Bill Daniel]

Stagpanther, I don’t know.
44 AMP yes and yes.
Thanks,
Bill

 

[stagpanther]

.02+ is a lot of "growth"--I would suggest using something like a comparator and modified case to get a real idea where your ogive engages the lands and back it off .01 give your bullet a moderate crimp and then drop your carrier on the bullet already in the chamber to see what happens. Creedmoor bullets by nature are "out there" beyond the case mouth--though I generally experience the opposite of what you are when I have a problem--the bullet can easily "hit things" on the way in to the chamber and get set back.

 

[44 AMP]

OK, so you chamber a round normally (using the closing force of the action) and the bullet slips forward in the case mouth until it is stopped by contacting the rifling??

If this is correct, I'd say you either need to increase neck tension, crimp, or seat your bullets deeper. Possibly all three.

Changing the force the bolt closes with, will reduce the force on the bullet when the case stops but will ALSO reduce the force available feeding rounds from the magazine. Reducing the speed of the moving parts below design specs can, and often does result in malfunctions.

Better you should fix your defective ammo than modify your rifle.

 

[Bill Daniel]

Adjust bolt carrier return force

Thanks fellows. My first approach was to increase neck tension. Bolt action have a recommendation of 0.002” between preloaded case neck and finished round. Zediker recommends 0.003” or greater with AR style rifles that is why I tried 0.005” but this did not change the bullet movement. I have measured the ogive to lands for these rounds with the Hornady modified case and as expected shows a similar forward slip.
Crimp maybe needed and that will be the next step if reducing recoil spring strength does not keep the bullet steady.
As always, thank you all for your help.
Bill

 

[44 AMP]

Bolt action have a recommendation of 0.002” between preloaded case neck and finished round.

I'm not understanding this so I think we're not quite on the same page with terms. What is a "preloaded case"? Do you mean unsized fired brass? Virgin brass? full length resized fired brass? Neck sized fired brass?

Where are you measuring the 0.002" difference? On the outside of the case?

Neck tension is the fit between the inside neck diameter of the resized case and the bullet.
Outside case measurement on the neck doesn't tell you anything but the size of the neck of the loaded round. Case neck thickness can vary, the inside dimension is more important than the outside dimension, provided the outside dimension is still small enough for proper clearance in the chamber.

How did you change the case neck tension by 0.005" ? By squeezing loaded rounds with a collet type die? Crimp die? or did you do something else? Resize the brass then load it? It can make a difference.

Also would like to know, do you have a bullet jump issue with factory ammo?
IF you don't then there's no issue with the rifle. Makes more sense to me to build ammo that will survive cycling intact than modifying the rifle to work "softer".

 

[jetinteriorguy]

I just had another thought. Lee makes an undersize mandrel for their collet neck sizing die. I have one and it definitely increases neck tension without crimping if that would be preferable. I did eventually find in my case it wasn't necessary for my loads, but it does do its job.

 

[stagpanther]

At some point in my many adventures with creedmoors in AR's I decided to try lighter force springs and lightweight carriers (you can get a graduated set from Tactical Springs). That can lead to a heavy recoil impulse and the BCG banging the buffer assembly pretty hard, depending on the bolt thrust. I eventually switched to rifle xl gas system length on my barrels and adjustable gas blocks, pretty much had to when going up and down in the range of bullets and powder charges. Creedmoor brass--I have thousands and probably all the different makes and I don't recall any of them--even after repeated firings--having a neck tension issue (except when they gave up the ghost and the neck split), though I usually put a light crimp on all my handloads.

I am very curious as to what is causing this issue for you--sorry I don't have much advice to help--mostly because most the cartridge COL issues I've had with creedmoors have been the opposite--something usually ends up crunching my cartridges shorter when I had a malfunction.

When I first got into hand loading I bought one of those inertial bullet removal hammers (my neighbors always wondered why I was out hammering on my front steps) and it's the closest equivalent I could think of to what you are describing. It holds the body of the case without support from the shoulder/mouth in order to get the necessary inertia for the bullet to come out. Considering the length of your run-out of the bullet from the case you must have something unusual going on IMO, possibly related to correct head spacing (even though that's a "usual suspect" in internet diagnosis).

I know a lot of people don't like crimping bullets

I'm not one of them--I crimp all of mine.

 

[Bill Daniel]

Again thanks to all.
The “preloaded in the case of the 0.023 bullet jump was virgin ADG brass which was full length sized in a Redding S die to an outside mid neck diameter to 0.287”. With a Berger AR hybrid 130 grain bullet inserted in the case the mid length neck diameter is 0.292” and 0.02” from the barrel lands.
Some multifired Shooting Star brass that I neck turned for uniform thickness (0.013”) with a mid neck tightness of 0.003” had jumps of just under 0.02” and even Federal Match with the same bullet moved 0.018”. Thanks for the clarifying questions.
As per Stagpanther if my plan fails I will try the Lee FCD.
All the best,
Bill