AR bullet seating depths-

[Quicksilver]

Has anyone else measured their bullet seating depths (against the bullet lands) in their AR's?

I ask because I can't actually get my loads long enough (due to magazine restrictions) to get my bullets anywhere close to the lands- I measured mine at 2.935 (which is pretty damn long IMO since the .308 is 2.800 OAL) but I load them at 2.860, as that's the longest I can reliably fit in the magazine.

I just really don't like my bullets having to "jump" over .100 in "factory" trim, and almost .070 in the longest form I can fit.

They shoot pretty damn well considering the distance that bullet has to jump, but I would really like it if someone out there offered a barrel with a "normal" throat length that could be maybe .050 or so from the "factory" established OAL.

Thoughts?

 

[Frank]

What kind of barrel do you have? What chamber. Sounds like its set up to run the 90 grain bullets loaded long single shot. There are shorter chambers. I have my gas barrel guy use my 700 rem 223 reamer. Mag length 68/69 & 75/77 only jumps just a little. 80s are a little longer and must be loaded single shot. Not a problem, the rules only allow single loading at 600.

 

[hunting1]

I just load 2.26 and have always had good accurracy and no feeding issues.

 

[Quicksilver]

ArmaLite AR-10T (chambered in .308, not .223) with a 20" SST barrel.

Definitely NOT setup to run a 90 grain bullet.

I checked it with a regular 175 SMK and empty brass case. Thought I was crazy when it came up 2.935, checked it 2 more times and came up with the same, had dykem on the bullet the 2nd and 3rd time.

 

[Quicksilver]

Guess nobody does precision or hot loads for the AR-10. Im just weird like that apparently.

 

[No1]

I wouldn't say that. While searching for "the load" the best I shot with mine was .110", 155g Amax over varget. But the load was too hot so I abandon that bullet. I shoot 168SMKs and Amaxs seated to magazine length. They do jump a long way but they don't seem to mind as long as they are in line.

 

[BCP]

Lots of factory rifles have long throats and shoot fine. Most match bullets will jump that distance and still make good groups.

Some of my best loads have been with seating depths that were quite short, in my DPMS SASS 2.78 with an AMAX was super accurate and in my last couple ARs 77smk @, 2.245 was great.

 

[BCP]

Guess nobody does precision or hot loads for the AR-10. Im just weird like that apparently.

Alright tough guy let's hear your hot loads and see some targets then.

 

[Quicksilver]

Alright tough guy let's hear your hot loads and see some targets then.

Don't get all emotional. Just thought it was odd nobody said anything in a week about the deeper throats and precision loads. Im trying to stick my bullet further out and develop a new load, but can't obviously start pouring different powder charges to sit around till I can establish a better bullet seating depth. I wonder if it really matters to spend 700 or so on a "custom" barrel with a shorter throat or if the accuracy is marginally different between a .100 "gap" and a .020 "gap" (bullet to lands distance)

As for "hot", I don't really know how fast its going. I have no chrono and truthfully, all I care about is average group size. If I want something faster, I'll put away my H4895 and dig out the Varget.

 

[LRRPF52]

Barrel time, OCW, projectile/neck/chamber concentricity are more important than seating depth. Even Berger has changed their recommendations on seating the VLD's based off of years of experience seeing that being close to the lands doesn't always equal the optimum accuracy in many guns in the 1000yd benchrest class.

Optimal Charge Weight, Longitudinal Shock Wave theory, and barrel time drive seating depth, not jump.

 

[Quicksilver]

Optimal Charge Weight, Longitudinal Shock Wave theory, and barrel time drive seating depth, not jump.

Could you provide informational links or definition/theory for this? I've honestly never heard of ANY of these things, but they all sound very interesting.

I assume OCW is finding what powder charge works well during a ladder test?
Never heard of LSW theory or BTD, but im guessing BTD has to do with how close the bullet is to the lands (I've always heard you want to be between .020 or .010 away)

 

[newbmowner]

I've measured several friends 308 AR type rifles for jump and it is not uncommon for them to have a .20" to .30" jump to the lands when loaded to magazine length. This is even on some "match" barrels from high end suppliers. Accuracy seems to be fine FWIW.

 

[Sniper Uncle]

Optimal Charge Weight, Longitudinal Shock Wave theory, and barrel time drive seating depth, not jump.

Could you provide informational links or definition/theory for this? I've honestly never heard of ANY of these things, but they all sound very interesting.

I assume OCW is finding what powder charge works well during a ladder test?
Never heard of LSW theory or BTD, but im guessing BTD has to do with how close the bullet is to the lands (I've always heard you want to be between .020 or .010 away)

Optimal Barrel Time Paper

Check this out, very informative. Works well with QuickLoad to figure out barrel times. If you want to, email me, and I'll forward some emails I had with Chris Long, the author of this paper.

 

[BCP]

Don't get all emotional.

I just thought it was pretty funny because there are lots and lots of people loading for semi 308s on this site and many of them are shooting very tight groups.

Seating depth comes up every so often and the consensus is always that they are worrying over nothing. Those same rifles with super long throats will often shoot 3/4MOA groups with retail match ammo which is about the best you can hope for from a run of the mill factory barrel and some shoot much better.

And no you don't have to spend 700 on a custom barrel there are plenty of drop ins for 400 or so that are great shooters. But without knowing what sorts of loads you are shooting, the groups you are getting and their velocities I have no idea whether you need a better barrel or not (you probably don't).

edit: here is a quote by the author of that paper linked above that sums up what I'm trying to say:

Interestingly, I could often take an optimum load with the bullet on the lands, and by seating the bullet deeper and deeper, pass thought a region of poor groups, until the group would tighten back up again. This ended up being another optimum, especially if I slightly reduced the powder charge enough to get the pressure/velocity back down to about the same as the on the lands case.

 

[Quicksilver]

After reading that I feel like all that I've done has been in vain.

So, just to recap what I thought he said-

Load (grains) doesn't matter except in the case of increasing velocity and filling the space inside the case-
Bullet depth is what really determines the tightness of grouping?

If that's true, why would a box of 60 bullets with the same seating depth and different charges in .2 increments disperse differently on paper? I tested some loads today and still can't figure out why the patterns were GREAT and very tight at 42.8 and 43.2, but the 43 grain was all over the place. Didn't make any sense, and of course I blamed myself for probably doing something wrong. I just thought it was extremely odd to go from under 1/2 to 1.5 to about 1/2 just by moving across .6 grains on range. Hell, I still think I must have done something wrong!

I really want to try going with a known good powder charge and change bullet seating depths in .010 increments to see just how much things change, because this is all new information to me!