Seating Depth for .308

[callgood]

I'm loading my first .308s, a 150 grain FMJ from Midway
My Sierra manual lists 2.775". I have mine seated to 2.770 since I get .002 to .003" jump when the round is chambered. I'm not crimping. When I seated the bullet, there is still (I estimate, didn't measure) about .0625" of copper showing between the mouth of the case and the bottom of the cannalure.
If I had been crimping the bullet to the bottom of the cannalure it would be seated to at least 2.70, maybe less. Would that have presented pressure problems?

 

[MCgunner]

When I'm working up a load, I first determine OAL by loading a bullet in a sized, decapped empty case. I load it really long and try to chamber it. Then, seat it a little deeper, try again. Finally, when I've got it seated deep enough so that I can close the action without the bullet's interference with the action, I'm good. I might then soot the bullet with a candle and close it in the action, withdraw it, and check for rifling marks in the soot. If I see rifling marks, I'll seat it just a little deeper until I can't see rifling marks. I then measure OAL and save the dummy round for future seating die adjustment. Of course, I make sure the cartridge will load into the action and cycle. It COULD be too long to fit if the rifle has a lot of chamber free bore, but I've never seen this in my guns.

I will then work up the powder charge by starting with a light load and working up a half grain at most at a time. I'll load up some rounds to be fired, five or ten, and take 'em to the range. I check for pressure signs at each increase in charge weight until I see a problem, then I back off half a grain. If I don't see any pressure signs I'll stop at the manual's max suggested load. The chronograph really helps to show me how the load is progressing, too, as I increase the charge. Some day I'll get an Ohler with a strain gauge so I can measure chamber pressures. That would be really cool.

I never crimp a load into a channelure except for 7.62x39 which is an auto and I'm a little worried about setting back a bullet on that one. In the auto's case, I'm less worried about freebore of the bullet and more worried about safety. If I have a little freebore when it's crimped in the channelure properly, I don't really care, especially since an SKS ain't exactly a bench rest accurate gun in the first place.

 

[callgood]

Thanks, MC. I should have mentioned I'm loading for a FAL, so the OAL listed in my manual fortunately coincided with my mags. I'm using Varget and my lightest loads pretty much fill to the base of the bullet. The heavier charges are slightly compressed. I haven't measured the chamber yet. If I ever get around to single loading, which I doubt at this time, I'll do it.
I've pretty much stuck to Varget for .223, and in all that time haven't detected setback, since my loads to date have been compressed to some degree. Generally get a little jump towards the rifling with .003" squeeze.
My question had to do with the fact I find it odd that if you DID crimp using the cannalure, the bullet would be " a lot" shorter than the manual. Would 2.270 or less be a problem re a change in pressure vs 2.277? I stick to the information in my manuals, so I have little experience with that amount of deviation.

 

[MCgunner]

I do have a taper crimp die for that 7.62x39. You don't HAVE to roll crimp into a channelure, but it sure makes me feel better in an autoloader. I'm not really too sure if my load is compressed or not, but I've never had a problem taper crimping it.

Sierra dropped my favorite bullet, though, a 135 grain "pro hunter" designed for .30 30 contender velocities. Bummer, that was a good hunting load.

 

[Ol` Joe]

The bullet you picture looks a lot more like a Hornady not a Sierra. Seating depth will be different if the brand isn`t the same as used in the book The load data likely will vary also. I don`t believe there is enough case volume taken in a bottle neck cartridge when varying seating depth to matter unless the load is already on the ragged edge.

There was a guy that posted over on a couple other board a few years ago by the name of OKShooter, aka Stan Watson. He did contract work in field artillery tactical fire direction program for the army and published a book of his finding on loading the 30-06 cartridge useing a Ohler M43 strain system.

In the book he shows a test where he altered the seating depth of a `06 carridge useing H4831 powder under a 180 gr Hornady SP. He found the pressures of the load varied from a low of 50,600psi with the bullet 0.164" from the lands to 53,400 with it jammed 0.021" into the lands. The pressures from the 0.164" to 0.001" (just touching) went from 50,600 to 52,100 psi or a change of 1500 psi with six various OALs total tested. The ES variation of the pressures are not listed although the SD of them is and runs from 600 to 900 psi so it is very likely the ES was probably close to, if not over the 1500 psi with most of these loads and may well have overlapped each others high and lows.

As for the canalure don`t fret them. The bullet manufactures put them at a point on the bullet that will allow when seated to them to fit most if not all SAAMI cartridges of that caliber and stay within the max SAAMI over all lenght for them when doing it. The crimp has nothing to do with how far they are from the lead as the manufacture has no way of knowing what the leade lenght in your rifle is. Most rifle manufactures vary considerably in them and as tooling wears they vary even more.

I don`t crimp any rifle cartridges except for 32 Spcl that will be fired in a M94 Win with the bullets linded up nose to base in the tubular mag. I doubt they would move still if not crimped, but I feel a little more comfortable with a crimp in them.

 

[USSR]

callgood,

Don't feel that you have to load to a specific OAL or have the cannelure into your neck. Load them out as long as your mag will accept, unless your chamber is shorter than mag length (not likely). In my bolt gun, I typically load the .308 to an OAL of 2.84".

Don

 

[callgood]

Thanks to all, some very interesting information. Once I chrono these loads and find the best group I may repeat the workup with a crimped series. I realize crimping isn't indicated for accuracy, but I would be interested to see if it makes much difference in a FAL. I neck size all my .223s for my Bushmaster and have had some good groups with 69 grain SMKs and 75 grain Hornady BTs.