Home Forums Hunting & Shooting Long Range Hunting Chasing Lands and Throat Wear

After owning my 7 Rem Mag for a few months and running probably 2-300 rounds through it, I believe I've started to toast the throat a bit... I'm just looking for some thoughts on this.

When I first acquired the rifle, I used my super scientific method of sticking the bullet in question into a fired case with enough neck tension to hold it, and closing the bolt on it to see where the lands were located. I did it 5 times, and all 5 times the measurement was very similar, and averaged out to 3.445".

The next couple hundred rounds I ran the rifle I seated at 3.44" after I found the 162 A-Max liked to jump 5k in this rifle.

After shooting a couple groups at 600 yards that seemed to be "double grouping" (3 in 2.5", the other 3 very tight, but about 7" away in a 6-shot group), I became suspect of my seating depth.

After re-doing my distance-to-lands measurement method, I came up with a new length of 3.464" to the lands.

Does this sound normal? Could I have moved the lands out .019" already? Or am I possibly seeing fluctuations in the ogives of different lots of bullets?

Welcome to the wonderful world of a round burning ~ 70 grains of powder.

You may want to invest in some tools. Hornady has a good OAL gage. I used to use the method that you are talking about, but am much happier now as I feel this is more consistent.

I expected to burn the throat, I'm just wondering if this amount of land degredation is normal or if it's another factor.

I would seat some rounds to .005 short off the new length and let the rifle talk to me. Whether you're burning the throat or something different, the most important thing is to fix the issue.

I've got 6 loaded up at 3.46" to shoot a few groups at 600 with. We're on the same page... With some luck they'll come back together tight.

One or the other, but impossible to say which without using the same bullet that you originally used to find the lands.

I'm no expert on the exact dimensions of throat wear, since I don't own a bore scope, but I think that 0.019" seems like a reasonable amount of throat to burn in 200-300 rounds. Especially when guys say that they see inches of throat burned out after a few thousand rounds of a hot cartridge like the 7RM. Although I'd find it odd for the ogive of the bullet to change that much from lot to lot. Seems like a lot. *grin*

Agree that the important thing is to get that load dialed back in, regardless of the reason for the change in distance to lands.

Tanner,

At about 2,100 rounds from my .223 the throat showed .019" erosion. That was with only 28 grains. I think your wear is right.

I suppose that's the glory of shooting a 2.5" case in a 3.6" mag... grin.

I also thought it was interesting to see my velocity decrease as the distance to the lands increased. I saw around .5 Minute of fluctuation at various distances with the same zero, but that could be any number of factors. Probably too many variables in that situation to say definitively, but it wouldn't surprise me.

Go to woods' post about the cleaning rod method a little way down the page.

It is just as accurate as the Stoney Point tool. Cleaning rod stops at Brownells for about 3$ a pair.

http://www.longrangehunting.com/forums/f28/measure-bullet-seating-50993/

It's possible that the throat moved back that much. I've notice more movement in the first 300 rounds than later in its life. It could also be a different lot of bullets.

Bb

You should be able to see throat erosion.
Looking from breech end, you should be able to see "frosting" of the leade.
Functional accuracy can go away pretty quickly, from there.

Could setting it back a couple threads and opening it up to a Mashburn clean up the throat??

Had the opportunity to look down my .257 Wby barrel with a Hawkeye borescope after about 300 rounds. Was quite "enlightening" to say the least...

John

IN reading your thread, especially the part about "double grouping" my first thought wouldnt be throat erosion. Double grouping is "usually", but not always,caused by a bedding issue. You didnt mention if your rifle was bedded so I had to wonder. Even if it was bedded, screws can come loose and things can shift. I have put literally thousands of rounds down range behind multiple rifles with 8-10,000 rounds without the double grouping issue, but when groups start shifting it usually means something is loose. But, every rifle is a rule unto itself. I hope its a simple fix for you/
Bill

I don't know if it's "normal" but last night I re-measured the distance to the lands in my 6.5 WSM after a similar number of rounds in a new barrel and got similar results. Someone said something above about quick "erosion" right off the bat in a new barrel, I was thinking it could be because the lands aren't perfectly square to begin with, maybe there's one that's a little longer than the others and it wears down fast. Only way to test that theory is keep shooting and see if the rate of erosion slows down.

Bill, the rifle is indeed bedded, but I will surely check everything for tightness when I get the chance.

I tend to blame the double grouping on seating depth because in the early stages of load development, I saw some big time double groups before I found the right depth.

I have to disagree, the Hornady OAL guage (formerly Stoney Point) is pretty much worthless. The way Tanner is doing it works much better !!!

I don't know about that. When I use mine I get results that are very repeatable. For wildcats I use Tanner's method though.

The hornady gage will get you close but by no means perfect. When you shove the modified case against the shoulder, I doubt that this dimension would be the same as your actual case against the bolt head.

I use them to get close and then seat a bullet a bit longer with just enough neck tension to hold it but yet force it into the lands. Doing this a few times as the OP says will be fairly accurate. To me, the key is to remove the firing pin from the bolt as well as the spring so you can really feel the bullet engaging the rifling.

This also works great for sizing a case for minumum headspace. The Hornady headspace gage works excellant for this, much better than the COAL gage.

I don't believe you would get double grouping if your barrel was on the way out. I burned out a barrel on a 25-06 AI once and it just started shooting larger groups with no grouping at all. When it went. it went a lot faster than I could ever imagine.