Seating depth: How well does it translate across propellants in the same rifle?

[bert2368]

While waiting for the "new toy" AR10 in 6.5 Creedmoor to cool between groups, I have been putting some previously made up test loads through other rifles to pass the time.

I dug out a dozen leftover loads for the 6.5x55 Swede CZ 550 with the 120gr. Barnes TTSX. The original loading with H414 was done at Barnes recommended OAL (3.000"). These shot over 3" for 3 a round group at 100 yards... And I set the rest asside, probably 2 years ago.






So I found them in a box yesterday, took the remainder and played with seating them deeper. I seated 3 rounds at each .02" interval SHORTER. And at .06" shorter than the original recommended depth, they suddenly grouped under 1".

It was 75 F. when I made that discovery. I hunt deer somewheres around +20F to -10F, usually... H414 is NOT well known for temperature stability.

I had a jug of H1000 on hand, and anecdotal information that it might work with the cartridge/projectile/environmental conditions I expect to hunt in. So, I set up a ladder test, starting a bit above minimum charge and WELL below Barnes recommended OAL, as this OAL had worked with the H414 loads.

What do you all predict... Is the length I discovered to be reasonably accurate with H414 likely to translate to another propellant?

I'll probably know the results by tomorrow around noon, but am curious what the more experienced people might expect my method to produce. I'm butt ignorant and trying to learn something of the basic principles here-

[GrocMax]

It will stay with bullet and barrel. Or more specifically bullet ogive and chamber throat.

Other factors with an influence would be bullet crimp and primer type.

[Oso Polaris]

Bert,

Its funny how shooting a bolt gun you are more in tune with rate of fire and barrel temperature. There is a psychological switch when you pick up an AR platform and suddenly the rate of fire is determined by how quickly you can acquire the target and the number of rounds in the magazine. Its the damnedest thing...I call it the "10/22" effect. To avoid this temptation I only bring 10 round magazines to the range.

Assuming you are talking about cartridges loaded with powders that have a similar burn rate, I think that the amount of jump required is primarily driven by the bullet profile. Fast powders (pistol and shotgun) with bullets that have a lot of bearing surface require more jump to avoid pressure spikes, whereas slower burning powders (rifle) with longer ogee bullets (minimal bearing surface) tend to favor short or no jump with less risk of high pressure spikes. For a hunting round the tolerance are a bit more forgiving then target shooting. I would be focused on selecting a powder that will handle your extreme temperature spreads, especially at the colder end during hunting season. Varget would be my starting point, but there may a better choice to handle the extreme low temperature.

[TinMan]

Oso, I do a similar thing concerning shooting ARs. I use only 20 round mags, usually GI aluminum and only load with 5 rounds, whether I am shooting a carbine or a 24" varmint rig. The "10-22 syndrome" is appropriate for just blasting, but I usually shoot for accuracy or groups, even if only on paper.

[Oso Polaris]

Originally Posted by TinMan

Oso, I do a similar thing concerning shooting ARs. I use only 20 round mags, usually GI aluminum and only load with 5 rounds, whether I am shooting a carbine or a 24" varmint rig. The "10-22 syndrome" is appropriate for just blasting, but I usually shoot for accuracy or groups, even if only on paper.

There is something about cycling a bolt that is meditative / therapeutic. I'll take 4 hours to shoot 50 rounds out of a bolt gun, but I have to force myself to go slow with an AR. I picked up the AR platform as result of participating in 3-Gun Competitions. Fast shooting with minimum of 2 rapid shots on each target from point blank out to 500 yards sometimes all in same stage. It reinforced a quick target acquisition and quick rigger response when shooting a semi-automatic platform.

[bert2368]

Originally Posted by Oso Polaris

There is something about cycling a bolt that is meditative / therapeutic. I'll take 4 hours to shoot 50 rounds out of a bolt gun, but I have to force myself to go slow with an AR.

Yes, it's like eating potato chips with the new AR-

I finished the barrel break in procedure yesterday, no longer needing to clean every 5 rounds-

And the round count per range session went up quite a bit. Put 20 rounds through the 6.5x55 Swede bolt action doing some ladder testing yesterday, but in the same session, 60 through the new AR...

Just for fun, I did a couple of repetitions putting 10 old beverage cans full of water on the berm and reducing all 10 to shredded scrap for recycling from 200 yards with 10 rounds loaded. Gratifying, but I need to concentrate improving my wind reading & mirage skills for (300 yards & up) accuracy. If I can only expect 2,500 round barrel life, make it count.

[Oso Polaris]

Originally Posted by bert2368

Yes, it's like eating potato chips with the new AR-

I finished the barrel break in procedure yesterday, no longer needing to clean every 5 rounds-

And the round count per range session went up quite a bit. Put 20 rounds through the 6.5x55 Swede bolt action doing some ladder testing yesterday, but in the same session, 60 through the new AR...

Just for fun, I did a couple of repetitions putting 10 old beverage cans full of water on the berm and reducing all 10 to shredded scrap for recycling from 200 yards with 10 rounds loaded. Gratifying, but I need to concentrate improving my wind reading & mirage skills for (300 yards & up) accuracy. If I can only expect 2,500 round barrel life, make it count.

When I shoot my bolt gun, I am completely in tune with the rifle from the easy of the bolt sliding forward to how much pressure to close the bolt to the easy of extracting the spent case and also the barrel heat. Take 3-5 shots and then let her rest. My precision 6mm has a 1500 round count life...I am painfully aware of barrel life. Luckily it is slow measured shooting.

My observation is that for the typical shooter at the range none of these lessons / habits are maintain when an AR Platform is being used. No one thinks about barrel heat. The 10/22 / Potato Chip mindset takes over...the rifle is made to withstand 20-30 round magazine dumps...right!!! The only time anyone notices the barrel heat is when it gets so hot that their hands start getting a little burned while holding the handguard. At my range I have watched guys bring a new case/flat of ammo (1,000 rounds) and go through all of it in one session... at the 50 yard line shooting a paper plate that eventually is nothing but the outer 1" rim of the plate hanging by a clothes pin. Load 30 round magazines and squeeze trigger every second until empty and then load the next 30 rounder...2 hours late they are ankle deep in spent brass that they throw away. It is shocking how few AR15's get stretched to 100 yards, lone enough 300 yards or more. It is dumbfounding!!! I have seen plenty of guys just shoot their AR 10's at 50 yards as well...

During the design/day dreaming stage of my AR10 build, I decided that I wanted an accurate rifle but the overriding priority was that it be utilitarian and only chasing medium range plinking (800 yard max). Zombie apocalypse and all that... I settled on .308 for the barrel life and availability of reasonably priced ammo (ignoring reloading in the decision process).

[TinMan]

A buddy is a great pistol shot, President's 100, 2700, etc. He loved doing 30 mag dumps in his 5.56, a Class III HK-53. Then he discovered that even 55gr GI ammo was keyholing at 50 to 100yds. Yup, the 8.5 inch barrel was fire/gas eroded for the first 4 inches. Eventually sold it for a fraction of its value. A hard learned lesson. Never got another Class III weapon.