Discussion about jump vs jam

[cosmic creepers]

So I was at the Range pulling a target and talking to a pretty mossy old dude.

He looks at my target, which has 9 sub MOA groups on it with a factory 1971 model 70 with a m118 chambered 7.62×51 24 inch M40A1 clone barrel on it.

Suggests that I shoot touching or 0.001" off the lands.

I suggested my reasons for avoiding the lands for pressure, load data would no longer be reliable, and I'd have to spend more components finding max for my gun, but I AM willing to do so if it really yields good results. I've had my very best luck off the lands, and in some cases 0.100" off the lands!

The closest I've ever been to the lands was .020" before the new barrel, and velocity suffered, and pressure was out of control. Got the pockmarks around the primer on my bolt face to prove it, which was bad enough I considered having a new bolt manufactured for it!

So what's the verdict here?

Is it worth doing for a factory .308 action with a 6 pound trigger in a wooden stock that gets shot off a bipod?

Will it be better than what I'm doing? And if so, enough for me to care?

I think group size is written on this target. Not all incredible, this was a seating depth test I was shooting, but only one of the loads shot a larger than 1MOA group of 5.

Thanks guys

-Phil [Pacific Coast

 

[BoydAllen]

The real question is whether you like to experiment and if you are really satisfied with your results? IMO one never gets serious about accuracy without a proper bedding job, which has two options, glued or pillared. With proper bedding, wood stocks shoot just fine. On the whole pressure thing, for me all that I need for load data is a safe starting load, at the bottom of listed charges. I do a one shot per charge pressure investigation, to determine my top load, with the bullet slightly into the rifling (.006). This prevents any pressure problems since I started so low and the seating depth is held constant. From there I go to a ladder test.

 

[MisterHelix]

I suppose it depends on your intended use. For me, a sub-moa .308 is good for what I need to shoot out to as far as I can send it, so another quarter moa doesn’t really do too much for me besides feel cool.

I don’t like to be too fussy. I’d rather load to mag length and drive on.

 

[Dimner]

I agree with boyd. I've stopped doing in depth load dev on any rifle that I'm not willing to do stock work on. Meaning, in your scenario, if you don't want to dive into pillar bedding, action bedding and a true free float of the barrel, I wouldn't really go any further than you already have. If you can consistently get 3/4 moa from a current load, I'd call it good.

 

[K22]

I'd take your rifle and load anywhere in North American on a hunt.

If you're just a recreational target shooter which I assume you are with the equipment you are using, then I would be satisfied with most of those groups. Anything 1 moa and under will please me with that equipment and shooting off a bipod.

Personally, I would never seat bullets into the lands or anywhere near them because of the variation of bullet ogives, I want to stay at least .010" off the lands. I don't want to risk a pressure surge or get a bullet stuck in the lands. But I am not a benchrest shooter so maybe that is needed in the discipline, I don't know.

Watch Ryan Cleckner's video on acceptable accuracy it might be helpful to you when you meet "experts" at the range.

Like BoydAllen said, the first step toward accuracy is addressing the bedding issue. All the load development in the world doesn't mean squat unless the rifle is properly bedded. Trust me, I learned the hard way to address this issue first before expending vast amount of time and money trying to achieve consistent sub 1 MOA groups.

 

[JFrank]

(Is it worth doing for a factory .308 action with a 6 pound trigger in a wooden stock that gets shot off a bipod?)

Can’t hurt ya to try a few depths with that bullet starting with touch and work out, you’ve already shot several rounds.

I’ve had rifles that were never bedded that shot amazingly well and others after bedding were not much better.
Go figure

 

[LHSmith]

Suggesting 0.001" off the lands tells me that old geezer doesn't know very much. Like K22 said above bullets from the same box can vary a few thou different in base to ogive lengths.

 

[jnyfeler]

Suggesting 0.001" off the lands tells me that old geezer doesn't know very much. Like K22 said above bullets from the same box can vary a few thou different in base to ogive lengths.

Exactly what I think when I hear people say to seat the bullet "just touching the lands".

Jeff

 

[cosmic creepers]

Hi guys thanks for all the replies.

First off, I can not iterate enough that the stock does not have any bedding issues that are going to solve my question. It has been glass epoxy bedded as well as it will ever be, unless I buy a new stock for it, which I likely won't unless this one cracks. I did have to take a lot of wood out of it to float this barrel.

Secondly, as for the accuracy, 0.010" sounds like maybe worth exploring for a quarter minute, but I'm not sure a consistent quarter minute can be had at my velocities.

So next question is, a ladder at suggested "jam" plus .010" is that expected to give me any additional accuracy past what I did, which was "jam" plus .020"

And what has been you guys experience with velocity. I am pushing 2575 fps with this load at .045" off the lands, would the increased pressure and thereby reduced charges cause my velocity to suffer at that jump, or can you guys usually ramp it back up to about the same per powder as having a reasonable jump?

Thanks again for replies, posting a photo of my final work up for the round as it sits.

I've selected .042" of jump [for now] and except for my fouling shot [i clean my rifle bore down to bare steel before every session], every group can be almost covered with a penny. Haven't measured them yet, but wanted to reply.

Thank you very much,
Phil

 

[cosmic creepers]

This is the target

 

[Ned Ludd]

Did the old guy look like this? You said he was pretty mossy.



The bottom line is that your rifle will tell you where a particular bullet wants to be seated. Not some old guy at the shooting range, not his moss, not someone commenting on a shooting forum, but your rifle. I generally do not like to seat bullets near "touching" the lands for a number of reasons, and blindly making such a claim at the shooting range after merely seeing a few groups would be a red flag for me. Nonetheless, if you wish to try Senor Moss' seating depth suggestion, why not just do a seating depth test from "touching" to -.021" in .003" increments? That type of test would seem to mate up with pretty well your testing above that started at -.024" and went deeper. Then you would know with certainty how the 168s shoot in your rifle when seated closer to the lands.

 

[Evan]

I’ve always been able to jam 0.020 or more without the bolt dragging on close (hard jam) or sticking and pulling a bullet getting the round out and I run very light neck tension in my BR guns. I think being fearful of going past the touch point is misplaced. A lot of people find fantastic accuracy with a jam and it doesn’t mean you’re going to stick a bullet or see pressure spikes. You just need to test and verify.

That said, if you’re happy with what you have then there is no reason to chase the rabbit down the hole.

 

[Bill Norris]

As mentioned above no one can honestly tell you what will work in your rifle. Only testing will give you the correct answer.

Placing a bullet in the lands is nothing to be shy of as long as the load is worked up from lower charges with the bullet in the lands. There is a difference between in the lands & hard jam.

 

[JFrank]

Looking at your recipe below, I’ve shot those 168 SMK bullets along with that powder several times in my own 308 , I always start charge ladders near the lands than work the seating out in .005 increments and normally find a seating node around .016 off. Point being ive not seen any adverse pressure signs starting on the lands.

This is the target