Why would a rifle shoot better with bullets jumped .200" rather than touching

[injuneer]

I have a 7 stw with 9 twist barrel. The closer I seat the bullet to the lands, the bigger my groups get. My best results are at approximately .200" of the lands? I have tried every everything from jamming .010" out to .200". The bullets I have tried are 160 accubond, 175 MKs, 168 and 180 Bergers. All have the same or simular results. The barrel is a Shilen and was done by a reputable gunsmith. My question is: Is it feasable to think that the chamber is not inline with the rifling or what? Has anyone else ran in to this.

Thanks,
Injuneer

 

[watercam]

There is no standard seating depth that always gives the best accuracy for any given barrel. My brother has a Winchester factory 30-06 that will only shoot with bullets seated 0.200" (not a typo, not 0.020") off the lands. It shoots extremely well there. I have thirteen rifles and most of them like the bullets just about .020" off. One likes them into the lands, one likes them .040" out, etc.. Even Berger recommends trying seating from .080" to touching in increments of .020" to .040" now. In the past it was thought that into the lands was the only way to go, but not every barrel is the same no matter who makes it.
If it shoots well, just shoot! You can check concentricity with the appropriate gauge if you want (most of us do on a regular basis), but accept your successes where they come.

 

[RonAKA]

Just a non expert opinion, but I think a jump is more tolerant to concentricity issues. I think a tight neck combined with a concentricity issue between the neck and the bore, and a jam could cause a bullet to be pushed out of concentricity before you pull the trigger. One possible way to test it may be to measure concentricity on a few rounds, chamber them with a jam, and then measure again to see if concentricity was affected.

 

[LongRiflesinc]

You have a big ol cartridge with a relatively small bullet.

A nice linear rise in chamber pressure where the cartridge is accelerating the bullet a bit prior to the barrel beginning to "bite" on the bearing surface may have something to do with it.

The alternative being the bullet jammed up in the lands and the case having to overcome the friction of the throat and releasing the neck tension. -That's more like a cartridge "sneezing" the bullet down the bore rather than a "cough".

With many magnums they seem to like the room to jump a bit rather than being squished up in the throat.

Hope this helped.

C

 

[smoking-brass]

I own a Rem 700 in 7mm stw and have tried different bullets and have found that 150smk's work the best .008 off the lands...it shoots a five shot group under .250 @ 100 yards. I'm in the progress of building another 7mm stw on a savage action with a varmint barrel...I 'll let you know how it shoots in the spring.
By the way I use imr 7828 with the 150"s...good luck

 

[sailhertoo]

You have a big ol cartridge with a relatively small bullet.

A nice linear rise in chamber pressure where the cartridge is accelerating the bullet a bit prior to the barrel beginning to "bite" on the bearing surface may have something to do with it.

The alternative being the bullet jammed up in the lands and the case having to overcome the friction of the throat and releasing the neck tension. -That's more like a cartridge "sneezing" the bullet down the bore rather than a "cough".
With many magnums they seem to like the room to jump a bit rather than being squished up in the throat.

Hope this helped.

C

He He........thats the best description of it I've ever heard!

 

[injuneer]

Thanks for the input. The quality of the responses in this forum are very good. Much better than some of the archery forums I have visited. I guess in the end I'm just not satisfied with the results I'm getting. This rifle is inconsistant and results varied from 1/2 to 1-1/2" groups. I have checked runout, turned necks, neck resized, FL sized, etc. I have other rifles that are consistant 1/4-1/2" groups without much effort. I had the gun rebarreled years ago and have tinkered with it off an on for a few years. Since then I have done a few rifles my self with better results. Before I buy a new barrel, I am going to set the barrel back and rechamber it and recrown. If this doesn't work, I'll start with a new blank. It'll be a couple of months to get around to it, but I will reply with the results. In the end my wife may have a new tomato stake in the spring.

Thanks again for all of the input.

Randy

 

[Larryh128]

Several manufacturers have throats long enough that you couldn't touch the lands if you wanted to. Some of these are quite accurate although they aren't in the benchrest league by any means. It is very possible that you have a chamber that is slightly tilted. If it is, it would give you the results that you mentioned. A barrel that isn't tight enough will do the same thing & sometimes you just get a barrel that plain won't shoot as well as you can. I would set it back & re-chamber, re-crown. If it doesn't perform as well as hoped. Sell it. A lot of hunters can live with a sub-inch barrel. Shilen makes great barrels & I have a couple that I wouldn't think of selling. I also sold one that I couldn't break 5/8" with after 3 different crowns.

 

[dakor]

I have a 7mm STW and I am jumping Berger 180's about .175 and it drives tacks same with a rifle my friend had and my dad is over .200 and his drives tacks also. I have loaded and shot several 7mm STW's I will give you a few suggestions. If you are using remington brass toss it in the garbage and get winchester brass. H-1000 is the powder of choice with a Fed 210 match or 215 primer. Can you give me the specs on your rifle and is it bedded? I tried moving my bullets like you did and it shot worse so I left it alone and the gun shoots half moa or better out to 1000 yards. I have a wyatts mag box in mine but my dad is using a standard Rem 700 Magnum mag box and his drives tacks like I said. I will also add my friends barrel and mine are Broughton 5c 9 twist my dads is a Schilen 9 twist.

 

[injuneer]

Thanks for the additional replies. After rereading my first post, I would like to clarify that if I could get a consistant 1/2" groups, I would be happy. It is when I get 1-1/2" groups the next time out, that I have a problem. Anyway, I have poped the barrel off and now have a pile of gun parts on my bench. I decided that my first step would be to replace the .200" thick lug with a .312" recoil lug (steping the barrel shoulder back by the .112") Pin it to the action and rebed the action with devcon. I will recrown and reinstall the barrel and give it a try. If that doesn't do it, I will pull the barrel off, step back the barrel a half inch and rechamber it. I get just as joy out of understand what the problem is as having a rifle that will shoot.

Randy

 

[DCRYDER]

Sounds like the chamber is a little off to me too. I have had a couple or factory rifles that have shown this when trying to "kiss the lands" with a smoked bullet. At one point i went from 6 light rifling marks, seat bullet .005" deeper, chamber it and it comes out with 3 rifling marks. Not good! This was done with ejector removed so it couldnt put side pressure on the case too.

 

[buyboysellboy]

Rifles are like people. All and I mean all are different. You can take 2 exact same model target rifles out of the box and they will shoot differently with their own likes and dislikes.

 

[clowdis]

You can pick a scenario (jumping .020, touching the lands, jamming .015) and tune your load to it and come up with an accurate load to fit that scenario. It might not be THE most fine edged accurate load that can be had for that rifle, but it will be a darn good accurate load. All you have to do is work at it a bit with different powders, powder weights, primers, bullets, etc. and you can find an accurate load. Sometimes there is no explanation about why a rifle likes a particular load. I don't like jamming a hunting load for any number of reasons, not the least of which is you probably can't get it in the magazine. So if your STW likes ammo with the bullet jumping .200 then I say it's fine, don't know why, but it sure doesn't create a problem if you're hand loading anyway.

 

[OldRifleman]

I have had all the problems you have stated with a 204 hunting rifle that was guaranteed to shoot 1" MOA.I did everything from different powders,different bullets,different primers,and so forth.Found out my bolt was touching the stock causing all the problems.Just thought I would drop this suggestion.Sometimes with me i can't see the trees for looking at the woods!Good luck with your problem.