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So Annoying! Bullet seating depth

quikcolin

Sergeant of the Hide
Full Member
Minuteman
Dec 16, 2018
157
69
London, Ontario
I spent about an hour getting the correct bullet depth .020" off the lands last night. Finally got the die all dialled in and decided I better check to make sure the ammo fits into the magazine. Good thing I did. I needed 2.2250 (with the Hornady comparator), I had to push the bullet down a mile for it to fit into the mag. All the way down to 2.200. The Hornady manual says the C.O.L should be 1.8000. I have it set to 1.8090.

Kills me. I wonder how much this jump to the lands is going to impact accuracy?
 
Case length is fine. Even if the 1.800 is max vs. trim to, most chambers are going to run on the long end.

If you are using pmags you can pull out the internals and run a file on edge down the inside front to allow for a longer tip. Making up 0.025 is pretty easy.

Chasing the lands is not a must for many bullet/barrel combinations. As stated, shoot them and see what happens.
 
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Thanks guys, I forgot to mention that. It's a 6.5 Creedmoor in an AR10 (Stag10). It is a Magpul Pmag as a matter of fact!

This is going to be a windy road I think. Did a wide spread of changes from the manual with H4580, from here I'll pick the best group and do increments of .3g on either side and see what happens!
 
In (Across the Course) Service Rifle, we shoot both mag length for rapids, and single feed for the 600 yards stage slow fire prone. Since that 600 yard line is slow fire, and we single feed, we can load 80 grain VLD designs that wouldn’t fit in the magazine.

Those exercises teach us two things. That properly reamed chambers do pretty well with the typical 69 to 77 grain magazine length ammo, but.... We definitely benefit from testing those seating depths on the single feed 80 grainers too.

I ran some factory loads with 77 TMK at the 600 yard line when I was traveling and didn’t have time to load. Do I give up some performance and windage? Yes. But not as much that I would give up and stay home.

Go try a seating depth development and you will see if that makes any difference to your goals. We can usually see two nodes on those 80 grain tests in an AR Service Rifle. If we start at jam and work down, the first node is usually within the first 5 to 15 mils, followed by another about 30 to 40 mils down. Sometimes being in those nodes makes the group really shrink, sometimes they are just enough to make you worry less about being perfected. When comparing the group size of an 80 grain load at the optimum load versus the factory or magazine length load, you will definitely see a difference. In general terms, the difference is in the X count. We are talking about a group size that would run maybe 0.5 to 0.75 MOA optimized, versus 1 to 1.25 if not. Calling the wind correctly is a much bigger effect and even a well centered 1.25 MOA group size would easily put you in High Master class.

So the bottom line is you should go test if you really want to know the impact. Even different bullet styles would require you to re run the seating depth development, so there is no way to predict how big a difference this will make.
 
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That sucks.

Honestly, the first thing i do is measure my mag and make a dummy cartridge to fit the mag. I'll then throw it in the rifle and see if the bolt will close and if the bullet has marks from the lands. If there are no makrs, and it's at max mag length, oh well, not much you can do about it either way. Unless you want to single feed in order get close to the lands... which I don't.
 
Thanks guys, I forgot to mention that. It's a 6.5 Creedmoor in an AR10 (Stag10). It is a Magpul Pmag as a matter of fact!

This is going to be a windy road I think. Did a wide spread of changes from the manual with H4580, from here I'll pick the best group and do increments of .3g on either side and see what happens!

6.5 creed loaded with h4580 with a coal of 1.8? Seems like a train wreck of incorrect information.