Old eyeball wrote:Well, disappointing number of participants... However, there does seem to be a suspicion, shared by several, that rifles behave differently when shot from the shoulder versus the test bench or vice. Having recently shot my rifle from a rigid, un-damped bench clamp, it's blindingly obvious that the two conditions are wildly different.
The strong vibrations set up with the rifle fired in the clamp can be felt to decay over about 1.5 seconds. No sustained vibration can be felt when shot from the shoulder. Group sizes significantly larger from the clamp.
So, the question remains - what can you tell by shooting from a bench clamp, if the rifle behaves differently when shot from the shoulder?
Hello,
I believe that stock mass changes vibration frequency and wave length.
Thus changing the barrel null point position. I further believe that the null point at the end of the barrel can be maintained by care full placement of the barrel weight and velocity of the projectile. Recently I made and installed a 5 pound weight as a fore stock. The added weight opened the group up greatly. Sort of like changing the inductive reactance in a tuned circuit throws off the frequency.
My thoughts
Chuck Marsh
shooting predominately FT with 12ft/lb there task of shooting 40 mm kills out to 55yds outdoors pushes the kit and ammo to more of a degree than perhaps 10m does...with the shooter being the 3rd part of the equation.
what we are finding, is that apart from ammo consistancy, enviromental conditions, particularly heat have a bearing.
one problem i've been working to solve is shift tied to temperature on walther dominators...having ruled out the gun's regulator and ammo by chrono testing, we still have found vertical shifts that were puzzling us. (over 2 MOA on some rifles with 20 degree drops)
looking at the interface between the action and the stock became the focus of our attention. One of the problems was that the regulator could touch the stock..which we cleared by milling out the area around the stock where it could touch...this solved some guns problems, but not all, suggesting there was another problem.
in the end, a close inspection of the bedding material pointed towards pressure still being exerted on the action, and shimming the action so there was no possibility of this happening seems to have done the trick. Unfortunately this means that the action can only be held in by one bolt and that the shiming has to be consistant across the other parts of the action...we solved this by fully floating the barrels, so there is no possibility of the shimming in A clamp being wrong, as it's been removed.
now this is probably irrelevant to 10m shooting which is in nice stable 10m indoor enviroments...but it does have a bearing on POI...and tests with torque on p70's have also shown that stock torque has an effect on group size.
there are also numerous reports of other guns displaying shifts as the pressure changes the stress on the air cylinder and transmits this into the action.
at 10m it's probably a very slight effect, if noticable at all, but I would say that it could be something to look into if that nth degree of consistancy was being looked for...however, I would say that if you couldnt get a decent group (well it should be one hole, lets face it) then it could be an area to look at.
the tolerences for shift were worked out to be in the order of 1/2 the thickness of a human hair to see a noticable shift.
funnily enough the p70 liked a very loose torque, almost to the point of rattling in the stock. But i must stress that this is testing at 55yds where perhaps variations are far more observable. It might make more of a difference with .22 LR shooting though.
we also have found that wooden stocked rifles dont show these shifts possibly because they are able to absorb the forces better or have different bedding points.
For what it's worth: Lock time on a standard Anschutz trigger, about 2.5 ms (0.0025 s). Barrel time, Tenex, about 2.5 ms. Not a whole lot can happen in that time. Adding weight to the stock probably changed the pressure on the receiver, which can change everything. The bullet is long gone before the stock starts to vibrate. No one can really tell you what bedding and action screw torque does, but if you get them right, the gun shoots better. I use a high durometer rubber bedding, torque now has very little effect. I set it to 5 N-M, per factory, but forgot to set it at one match, and the gun shot fine. At another match, one of the Belleview washers on the rear screw broke and I had to back that screw down to 2 N-M. Gun still shot fine. Wish I could say the same about the shooter...
Also, frequency of vibration: about 3 kHz. Amplitude ? Mighty small. There are much lower frequency components, they don't appear until long after the bullet is gone.
I have a number of oscilloscope photos of this, but other than being 'interesting', we didn't find them very useful. I use the Henrich Vibration Controller rather than a tuner, mostly because I'm too lazy to adjust it for different conditions of temperature, humidity and ammo lot. It helps on some guns.
I shot smallbore in the late 70's early 80's.....i never remember this level of torquing, ammo testing and tuning. you bought an anschutz, a case of eley and shot your matches. it sounds like an awful lot of time, testing, frustration etc. what kind of groups at 100 yds are you all looking for??
mark
groverdog1 wrote:I shot smallbore in the late 70's early 80's.....i never remember this level of torquing, ammo testing and tuning. you bought an anschutz, a case of eley and shot your matches. it sounds like an awful lot of time, testing, frustration etc. what kind of groups at 100 yds are you all looking for??
mark
3/4" max group size at 100 yds is what I strive for...and (gasp) I test from the prone position. You can tell how ammo performs by using a scope in reasonable conditions. I have never seen a group shot on a rest/cradle that was not able to be improved when the ammunition is tested from the prone position.
With the cost of a case of Eley (or any other top brand) at over $1100, it's a no-brainer to test lots before you buy. Tuning is a new gimmic from the benchrest crowd and has, in my opinion, only received luke warm results in prone & 3p due to repeatablity of the tuner settings due to variation in the human support position. In addition, the significant weight of a tuner can play havoc on a position by making the rifle extremely front heavy.
Torquing the action screws is essentially tuning the rifle via the action and not a lump of weight on the barrel. This is more of an advantage w/ wood stocks, as temperature and humidity can significantly change the wood to metal interface.
I tried a tuner for a good year or more, and found that while you CAN get it to work, it's just not conducive to international style shooting. (Try fiddling with a tuner without breaking your position). We have enough to worry about out there trying to keep 'em all inside that little bitty-ass 10 ring. NRA maybe, where you can go back to the sighter and you get breaks every 20, but not in ISSF type events. Plus the damn thing makes the gun barrel-heavy. So then you're left asking yourself, do I leave it on and produce another potential set of problems that I can't deal with easily during a match, or do take it off for some matches and put it back on for others-- and end up with different harmonics?
I've found that my gun shoots best with a very specific ammo speed, and the lots that I've found at that speed that have the smallest S.D. in testing in fact shoot the smallest groups and the best scores in real matches. This backs up a previous post that alluded to the speed itself being a critical factor, and the consistency being the other. My experience bears that out. My experience also bears out that practice is more important than any other one factor. I've monkeyed with every techie thing out there, but if I've not trained long and hard, I'm not doing the ammo justice-- I'm wasting it.
So test if you want, and find that killer lot, but it's pointless if you don't train your butt off.
One person mentioned using noptel and relating the group to the trace. That is worthwhile. I believe I can do almost the same thing by calling my shots, ploting the cals and relating them to rhe hits on the target. I have seen good accurate results from some machine rests but to get them took almost as much zero tolerance work as it takes to tune a rifle properly. The last posts make a lot more sense to a shooter than all the super intelectual self agrandizement contained in many of the earlier posts Just my humble opinion, I'd rather spend my time on the range. Good Shooting Bill Horton
I have been working on a project and ran across your threads in this forum that parallels the issues I have been working on. I am looking for some advice. I started my project with the intent to index a barrel. Barrel indexing has been thoroughly discussed in the Smallbore Journal on the Delphi forum. Without having a 2013 action that allows switching and rotating barrels and is favored currently for indexing I attempted to use a barrel block mount. I found with the receiver free floated that the accuracy was compromised and would not reliably provide group data fired at 50 yards for an index location of the barrel. In playing with this setup I found that with minimal torque on the action screws, and especially the rear screw I could readily bring the group to .175 CTC. This is a very flimsy action as I used an Anschutz 64 MPR for my setup. Have any of you had experience with barrel mounting? Am I on a fools errand to attemp accuracy improvement through barrel mounting? Would you have any suggestion for my continued effort? Thank you in advance for any help you could provide. Jeff
(Previously posting as Old eyeball - now I'm a registered member!)
Jeffrey, exactly what are you hoping to do? It sounds interesting, if I've interpreted you correctly. Can you give us more detail?
It's been suggested, (harold Vaughan: Rifle Accuracy Facts) based on evidence, rather than belief, and backed up by good engineering theory, that a heavy, but flexible barrel is less perturbed by induced vibrations, than a light, stiff barrel. The research looked also at barrel-to-receiver mounting rigidity, how some designs of joint were critically unloaded by forces caused by the explosion, and also looked at the undesirability of asymmetry in the receiver and how the recoil force was coupled to the stock. Do you know this book?
It seems to me that (if this is what you're doing), a rigid coupling of the barrel to a rigid, high-inertia foundation, may produce similar effects to a lighter, more rigid barrel. When the barrel is cantilevered off a fairly flimsy receiver, which is fairly compliantly bedded (the compliance being alterable by the mounting screw torque), you have something nearer the desired heavy, but floppy barrel.
I was attempting to devise a method of indexing my barrel radially to it's optimum position for accuracy. I began buy mounting the barrelled action using a barrel mounting block just ahead of the receiver in a test stock. I was disturbed with the result as the group size and shape were erratic. I started playing around with it and found I could "tune" this system at the receiver to perform better than it had performed in the original stock with the conventional mounting system. Using an Anschutz 64 action MPR I was not sure what improvement, if any, I would obtain but due to the flimsy barrel/action juncture I believed I would see changes without sophisticated instrumentation. I have been successful in getting the rifle to perform to a reasonable level that is an improvement from the "as factory" configuration. As factory, the gun performed in the mid .300 inch range and now can repeatedly in the low to mid .200 inch (5 shot groups at 50 yds). I realize there are many who own this model of rifle an claim under.200 inch consistently. I think there is a little "cherry picking" going on as I don't think the MPR set up is that capable. If it were I would trade my 2013 for one. I have not read the book you referenced, unless it was a long time ago and my aging brain forgot. As far as asymetrical actions not being optimum for performance, my jury is still out on that. Thanks for the reference, I will attempt to find the publication. I have to believe others have tried this and I'm looking for "the other shoe to drop".
