Bolt Lug Contact & how it effects groups

[gonehuntingagain]

Ok guys, I searched the site to try to find my answer, but didn't really find what I was looking for. After trying some 180gr Accubonds last weekend with my Rem 700 VSLH, I was a little dissapointed to say the least. I was using Varget, Win brass (prepped, but not sorted by weight) with Fed 210 primers.
There were a few funky things happening. Up until the last loads (44.8 and 45gr (max listed load)), the loads seemed to string up and left - I didn't look at the target after each shot, only after 3 shots would I check the target. When I got to the max listed loads, the groups seemed to lose the up & left trend and would shoot a triangle shaped group. The load w/ 45gr shot a 3" group. All shots were taken at 300 yards, and I caught one of the few days of the year that there was little to no wind. It didn't seem to matter if I waited 5 min between shots or not. The action is skim bedded, and there is no interference between the stock and barrel all the way to the recoil lug - I can fit a business card in there.

The 45gr load velocities were 2652.5, 2650.7, 2639.3 (chrono was a Pact MK IV 6' away from the muzzle).

I thought I read a post by Kirby that said bad lug contact would cause more issues as the pressure got higher, but with my rifle it doesn't seem to be the case, just a whole lot of wierdness. How much does the bolt lug contact play a role in the rifle's accuracy? Would it be worth it to have the receiver trued, and have the factory barrel chamber recut so it is not the sloppy factory Remington chamber (money considerations here - not able to buy a new barrel at this time in addition to having the receiver trued).

Here are some pictures to illustrate my issues.

Left bolt lug - the shiney spot was not form the flash - it appears that only a small portion of the lug has been making contact.

Right bolt lug - small contact patch as well.

Stringing. For some perspective, the sticker on the target is 3 7/8" wide, 1 7/16" tall.

This one was with 45gr Varget, about 3.5" at the widest point.

Maybe my expectations are a little high for a factory rifle...

 

[royinidaho]

GHA,

I sure don't think you're asking 'too' much of a factory rifle. But it does look like you have a bit of a fun project.

I wouldn't think that the bearing surface on the lugs would make "that" much difference but I wouldn't want to live with the way they mate up, either.

I'm thinking that there is much you can do to improve things without getting into the action truing thing.

You could lap the lugs yourself without taking the barrel off. I'd tell how I'd do it but would rather someone chime in that may have a bit more sophisticated approach. Though I would say that you would want to take the bolt apart.

The grinding compound is time consuming to clean up after the operation is complete when the barrel and action are assembled.

The resulting few thou increase in headspace shouldn't be a problem as long a you are aware of it.

The first thing I'd check is the amount of barrel movement that occurs as the front and rear mounting screws are alternately loosened and tightened.

BTW, this is the kind of project that I thrive on and tend to get a little more wordy than usual /ubbthreads/images/graemlins/frown.gif But this is where one can gain a bunch of useful experience.

Good luck & keep us posted. BTW how much snow did you get this morning? We got dumped on over here /ubbthreads/images/graemlins/confused.gif /ubbthreads/images/graemlins/confused.gif

 

[Fiftydriver]

With an opposing bolt lug design like most modern bolt actions such as the Rem 700, uneven bolt lug contact will usually present itself in vertical stringing. Generally this stringing increases the higher the pressure gets in the load but this is when one lug contacts solidly and the other is floating.

as the chamber pressure increases to the point where the bolt will be flexed this is where the stringing begins to get severe in some cases.

With your rifle, its hard to tell from pics but it appears that only two small portions of the lugs are contacting which dies seem a little odd just from a machining point of view.

If this is the case you may see inconsistant fliers in all directions as you are seeing. I would get the lugs trued and lapped and see what that does for your consistancy.

One note of caution, if this area of the rifle is this poorly fitted, it may be just the start to the problems in the rifles machining. I would not dump alot of money into this rifle unless you are going to rebarrel and accurize the receiver and make sure the machining is correct from the start.

Remember the rifle has alot more resale value as it comes from the factory so if some simple cures do not solve your problem I would recommend that you either decide to rebuild the rifle correctly or trade her off for a new stick before you loose the resale value by converting the rifle.

If this the only bullet you have shot in the rifle? Is there a pattern with other bullets?

The 308 should not be that hard to get to shoot well and while your playing with 1 moa, its not really want your looking for in a rifle like this.

Good Shooting!!

Kirby Allen(50)

 

[gonehuntingagain]

[ QUOTE ]
With your rifle, its hard to tell from pics but it appears that only two small portions of the lugs are contacting which dies seem a little odd just from a machining point of view.

If this is the case you may see inconsistant fliers in all directions as you are seeing. I would get the lugs trued and lapped and see what that does for your consistancy.

One note of caution, if this area of the rifle is this poorly fitted, it may be just the start to the problems in the rifles machining. I would not dump alot of money into this rifle unless you are going to rebarrel and accurize the receiver and make sure the machining is correct from the start.

Remember the rifle has alot more resale value as it comes from the factory so if some simple cures do not solve your problem I would recommend that you either decide to rebuild the rifle correctly or trade her off for a new stick before you loose the resale value by converting the rifle.

If this the only bullet you have shot in the rifle? Is there a pattern with other bullets?


[/ QUOTE ]


Thanks Kirby. I do not intend on selling the rifle, mostly because it is a real pain in the rear to find a left handed rifle, and then who knows what I would end up with. I would like to have it trued/blueprinted (eventually). I have been shooting Nosler 168gr J4's using mil-surp WC844 powder (not the most consitant stuff in the world) and have not seen a pattern like this.
I have also tried IMR 4350/168 J4s with the rifle to see what it would do and it wasn't too bad.

You saw the bolt lugs correctly, they both appear to be contacting with about 1/3 or less of the bottom of each lug. I know it would be well worth my money to have the receiver trued, but would it be worth it to have the barrel set back and rechambered true, assuming the bore is centered in the barrel (since I don't have the funds available for a new barrel)?

Since the groups started to get better at higher loads, and there were NO signs of pressure issues, I am going to try a couple of loads at .1gr and .2gr over max listed just to see if the group tightens up any.

I thought of doing the valve grinding compound or JB bore scrub approach to lap the lugs, but I figured that it would be only a band-aid to the problem. Heck, maybe my rifle just doesn't like Varget and/or the 180gr AB's

Roy - it tried rather weakly to snow Sunday morning for a few minutes then all you had to do was wait 5 minutes and the weather would change - all day long...Today added a little hail. It looked like it snowed like mad in the mountains though. One ski resort already announced they had enough snow to stay open until late May. If the weather gets good this weekend it will be time to blast some chucks.

 

[Fiftydriver]

Gonehuntingagain,

If the rifle likes the 168 gr bullets then I would say what I say to all of my customers, "A rifle is like a beautiful women, give her what she likes and your life will be great, give her what "YOU" like and things will go bad quickly!"

Translated to rifle talk, shoot the bullet that the rifle likes and don;t bother with ones YOU want to shoot.

It may simply be an issue with stability at the velocities you are getting with the 308 and the 180 gr accubonds. May also be some issues with bore diameter being a bit loose and the solid head of the Accubond not bumping up to seal the bore which can cause some odd fliers at times.

Again, if this is the case, higher chamber pressure will generally help tighten groups because it kicks the bullet base harder with higher pressure and bumps it up more.

With the Accubond though, its solid head prevents most of this bump up so you basically get what you get, not like a cup jacketed bullet.

I would still recommend lapping the lugs but I much prefer to pull the barrel and do so. This way you can keep an eye on the amount you set the bolt back for headspace purposes.

I would not lap off more then a couple thou at the most. You can take this amount off and still not have major issues with headspace in most factory rifles. Most are so darn loose it hardly matters anyway but I do not recommend going over 2 thou lapping the lugs unless you will set the barrel back.

2 thou off the lugs is actually quite a bit. You will see most lugs come into at least 75% contact with 1 to 1.5 thou lapped off the lugs.

As far as if it is worth it to set the barrel back and rechamber, thats a personal decision, professionally, I do not like recommending that proceedure for one simple reason, your still left with the factory barrel.

If the only thing wrong with the rifle is that the factory barrel is the problem with accuracy, you can bump $500 into the rifle to true up machining and still be left with results pretty much the same as when you started.

Sometimes the results are very good, most times there is some measurable improvement in consistancy but there are other times where nothing improves using the factory barrel. Its hard to tell your customer to pay his bill when the rifle shoots the same as it did when he brought it to you!!!

For this reason, I generally only recommend minor things when the factory barrel will be retained on the rifle. Lapping lugs slightly, bedding, recrowning, trigger jobs and such are easy to do and generally relatively inexpensive as well.

Much more then those steps and I really recommend investing in a quality barrel and giving the rifle a total makeover. The improvement in performance to the cost in dollars increases DRAMATICALLY with a new barrel and a fully accurized receiver.

Yes it costs more, several hundred dollars more, but in the end, I have yet to have anyone complain about a properly reworked rifle.

Simply put, if your 308 likes the 168 gr bullets, listen to her!!! Rifles are very blunt with what they like, especially factory rifles, give em what they want.

Kirby Allen(50)

 

[gonehuntingagain]

Thanks Kirby!

I'll play around with some differet bullets and see what I come up with. I am trying to get a good hunting load worked up for Mulies, and the 168gr Nosler J4 doesn't cut the mustard. I killed my deer last year with them, and was VERY unimpressed with their performance. I still have some 165 gr ballistic tips around, so I will give them a try if the 180's don't pan out.

The rifle will eventually make the trip to Montana to get some TLC. /ubbthreads/images/graemlins/wink.gif