Reloading for semi-auto: 260 Rem.

[Thebelly]

I'm pretty new to reloading.

I can't get the gun to reliably function. The two loads I've tried have excellent accuracy, but they fail to pick up the next round (and also lock the bolt back on the last round).


The gun is a LMT MWS w/ a .260 rem 20" SS barrel.

I've tried two different loads so far.

Bullet: 120gr A-Max
Brass: mixed, .308 necked down.
Primer: Remington 9 1/2 large rifle
Powder: Varget 36.8gr and 37.1gr.

I'm a little paranoid about over pressure.

Here's my question: will a slower burning powder (like h4350) help or hurt the reliable functioning issue? Maybe I should just increase the charge a bit?

Again, Im paranoid about over pressure in the gun.

I appreciate any help.

 

[FCS]

AR you have to worry about peak pressure, port pressure and gas volume that will be cycling the BCG.

H4350 will help cycling as it sounds like you are short stroking. H4350 will have lower peak pressure but increased port pressure and gas volume.

Mixed 308 necked to 260?
What brands of brass are you running?
What is loaded round measuring for neck diameter?
What is a fired case measure for neck diameter?
You might have a clearance issue that can mask / emulate other issues.

What is OAL to touch lands 120 Amax?

Varget with 120s you should be fine but a touch slower burn rate wouldn't hurt you any. I'd say you are a little light on charge but without seeing primers, case head expansion, etc it is really hard to tell from here.

Why 120s vs 139/140?

 

[Thebelly]

AR you have to worry about peak pressure, port pressure and gas volume that will be cycling the BCG.

H4350 will help cycling as it sounds like you are short stroking. H4350 will have lower peak pressure but increased port pressure and gas volume.

Mixed 308 necked to 260? Yes
What brands of brass are you running? Once-fired LC and once-fired BH
What is loaded round measuring for neck diameter? .300
What is a fired case measure for neck diameter? .300
You might have a clearance issue that can mask / emulate other issues.

What is OAL to touch lands 120 Amax? Unknown, I have to load to shorter than mag length. I load to 2.780"

Varget with 120s you should be fine but a touch slower burn rate wouldn't hurt you any. I'd say you are a little light on charge but without seeing primers, case head expansion, etc it is really hard to tell from here.

Why 120s vs 139/140? The twist rate is 1/9

Answers in bold.

 

[Rhyno]

Hmm, those neck diameters worry me, quite a lot.

What you need to do, immediately before you fire another round is call whoever built it and get a measurement for the neck diameter on that reamer.

You are probably going to have to turn the necks. If you don't have enough clearance around the neck you will get a stuck case, or much much worse.

For an autoloader, .004 neck clearance minimum seems to be recommended with some going a bit higher, especially if you want it to function in all kinds of environments.

 

[Thebelly]

Hmm, those neck diameters worry me, quite a lot.

What you need to do, immediately before you fire another round is call whoever built it and get a measurement for the neck diameter on that reamer.

You are probably going to have to turn the necks. If you don't have enough clearance around the neck you will get a stuck case, or much much worse.

For an autoloader, .004 neck clearance minimum seems to be recommended with some going a bit higher, especially if you want it to function in all kinds of environments.

So, all those terms are new to me.

The neck clearance thing, how would that manifest itself (stuck cases, hard to manually extract, etc)?

My interpretation is that the neck diameter in the chamber should be a bit larger?

I apologize for being mr. Newb, but I'm just too new to understand what the 2nd order effects of what I've got going on right now.

(I've stopped everything for like two weeks, as I've got surgery later this week.)

 

[sawgunner2001]

So, all those terms are new to me.

The neck clearance thing, how would that manifest itself (stuck cases, hard to manually extract, etc)?

My interpretation is that the neck diameter in the chamber should be a bit larger?

I apologize for being mr. Newb, but I'm just too new to understand what the 2nd order effects of what I've got going on right now.

(I've stopped everything for like two weeks, as I've got surgery later this week.)

The fired cartridge neck outside diameter *should* be larger than the unfired cartridge neck outside diameter. This is for reliability in loading, but more importantly for safety. If you get a cartridge that just barely fits in the chamber and is slightly forced in thereby pinching the bullet inside the neck (especially on an auto loader), you can run into HUGE pressure issues.

The solutions to this problem are: use brass with thinner necks that measure less than .300" when loaded OR remove material from the outside of the necks of the brass you're currently using OR have a 'smith ream the neck area of your chamber a couple thousandths larger. Rhyno's suggestion of calling the smith who chambered the rifle and determining the diameter of the neck of the reamer he used will confirm/deny the neck measurements of your chamber. It is my opinion that this issue is unrelated to your rifle's failure to pick up a round in the magazine problem, but a problem nonetheless.

 

[TimK]

Belly,

For reference, Lapua brass measures .294" loaded. I had Hoser make me some brass from LC .308, and he had to turn the necks down. I agree that .300" is probably quite close to the chamber dimension.

I can loan you my neck turner if you need.

Tim

 

[Thebelly]

The fired cartridge neck outside diameter *should* be larger than the unfired cartridge neck outside diameter. This is for reliability in loading, but more importantly for safety. If you get a cartridge that just barely fits in the chamber and is slightly forced in thereby pinching the bullet inside the neck (especially on an auto loader), you can run into HUGE pressure issues.

The solutions to this problem are: use brass with thinner necks that measure less than .300" when loaded OR remove material from the outside of the necks of the brass you're currently using OR have a 'smith ream the neck area of your chamber a couple thousandths larger. Rhyno's suggestion of calling the smith who chambered the rifle and determining the diameter of the neck of the reamer he used will confirm/deny the neck measurements of your chamber. It is my opinion that this issue is unrelated to your rifle's failure to pick up a round in the magazine problem, but a problem nonetheless.

This is my eff-up. I didn't use the calipers correctly. After a quick lesson via youtube, I re-measured correctly and here are the results:

The neck, closest to a loaded cartridge:

The diameter of the neck right next to the shoulder is only about 0.001" larger.

I don't have any fired cases, as 'brass prep' is one of my clean up steps from range time.


TimK: you truly are a good man, and a heckuva golfer!! I'll probably end up buying my own this week, so ill have about 437 questions to ask about how to use it.

 

[Rhyno]

It's still to large IMO, though you may need a better caliper.

The SAAMI spec is .298 for a 260 Remington.

You'll need to call the Smith and ask what he reamer he used and get the neck spec. If you use a case with to thick of a neck it'll lead to, failure to feed, over pressure, and stuck cases, possibly ripped off necks or case heads.

 

[Danco411]

Agree. In a bolt gun you may get away with not turning resized .308 brass for the .260. Worse case is a sticky bolt. In an AR where gas pressure, buffer and spring all balance for proper extraction anything off can cause a short stroke. Another question, is this a middy or rifle length gas tube? Adjustable gas block? Even a slight oversided neck thickness will cause just enough tension on extraction for short stroking. Have you tried factory ammo? If the factory ammo cycles fine then you either need to use .260 remington brass or turn your .308 necks. If your still short stroking with the factory ammo then you have a gas issue with the rifle.

 

[Thebelly]

It's still to large IMO, though you may need a better caliper.

The SAAMI spec is .298 for a 260 Remington.

You'll need to call the Smith and ask what he reamer he used and get the neck spec. If you use a case with to thick of a neck it'll lead to, failure to feed, over pressure, and stuck cases, possibly ripped off necks or case heads.

Just because I'm confused a bit..... If SAAMI spec says it needs to be 0.298", and mine is 0.294" (loaded), how is this still too thick?

as its a factory 20" SS barrel, I'll call LMT and ask about the actual dimensions that its reamed to. Non-adjustable gas block that I'm tracking.

After I load up a batch of five, I test load them in the gun, and they load and unload easily. It doesn't feel like any cases are stuck in there when I pull the charging handle to unload them.

In any case, I really appreciate everyone's words of wisdom. I've got a couple weeks of non-shooting to really dial things in. Hopefully I can get it all figured out so that the next range session is full of win.

 

[Rhyno]

Well, it might be okay, I error on the side of caution.

I don't view a dial caliper as a precision measuring tool, even when they read in .001, yours reads in .01, there's just a lot of room for error.

I would see if you could find/borrow a better measuring tool.

 

[Thebelly]

Well, it might be okay, I error on the side of caution.

I don't view a dial caliper as a precision measuring tool, even when they read in .001, yours reads in .01, there's just a lot of room for error.

I would see if you could find/borrow a better measuring tool.

meh, I'll just buy another. two-is-one, one-is-none, right?

digital calipers should be around $100.... any favorite brands?

 

[Rhyno]

No, you might ask someone else, I got a digital John Deere one I side for COAL, and base to ogive, then a Snap On dial one for everything else.

 

[Thebelly]

You'll need to call the Smith and ask what he reamer he used and get the neck spec. If you use a case with to thick of a neck it'll lead to, failure to feed, over pressure, and stuck cases, possibly ripped off necks or case heads.

LMT wouldn't tell me the specs for their .260 barrels. My next question was, "Will SAAMI spec ammo work in this chamber?"

The response I received was, "Buy a box of factory ammo and if yo ustill have problems, call back."

It didn't take long, but that wasn't exactly what I was expecting to hear.

 

[Rhyno]

Yikes, well Saami spec for a loaded round is .297 maximum.
If a fired round is measuring .300 then their chamber spec is a little looser then .300

I don't know what to tell you really.

Though this neck clearance was more about safety, it might not have a lot to do with your original issue of not picking up a new round, which sounds like a short stroking issue.

 

[Thebelly]

Yikes, well Saami spec for a loaded round is .297 maximum.
If a fired round is measuring .300 then their chamber spec is a little looser then .300

I don't know what to tell you really.

Though this neck clearance was more about safety, it might not have a lot to do with your original issue of not picking up a new round, which sounds like a short stroking issue.

Yeah, I got the 0.300" from my retardedness at using a caliper. I flat-out measured incorrectly.

I also don't have any fired casings, as I tumble and re-size the brass as soon as I get home from the range: tumble, clean guns, resize brass.....

 

[Rhyno]

Okay, well in my experience measuring fired brass doesn't tell you much anyways, my fired brass was only .001" larger diameter then my loaded brass. But brass shrinks I actually had .005" of clearance.