@ 04:16 pm (GMT)
Nathan FosterHi Gary, looks like the rifle is nearing the end of the options line. If the barrel is fouling severely, it may have a burrs within the bore of a mal formed land. I have also seen bores that are tight at the chamber end of the barrel, then loose at the muzzle, causing a near explosion of copper vapour which ends up guilding the entire bore. If the bore is the problem, you have one final option and that is to fire lap the bore. Give the rifle one more try and if it is still playing up, either sell it off cheap as a donor for another shooters potential project- or rebarrel.
I do need to ask though, what amount of time was given between firing shots. The trouble is, if the rifle was fired quickly during the run in phase with the suppressor mounted, the suppressor could have caused problems. If the bore is allowed to heat right up, the heat will expand the steel, reducing the bore dimensions. This can cause severe copper fouling up to the point where the bore eventually yields to the pressure of the projectiles being forced through it, expands (hot forms) and once it cools, becomes oversize. Once the bore is oversize, the rate of fouling will be reduced but at this stage its goodbye barrel, groups are out around 3. I am seeing this more and more now. Suppressors are a great tool but they can ruin a rifle.
A major problem is that the suppressor manufacturers fill their websites with why you should buy a suppressor, but never tell readers how a suppressor should be used and the potential problems of mis-use. That would be like me selling you bedding compound, telling you to throw a dollop in the stock and shell be right without worrying about critical points of relief, as if a squirt and a squelch is going to fix everything including a bore problem.
Carbon caking at the chamber is another problem with suppressed rifles. The carbon reduces the neck diameter of the chamber so that in effect, when a cartridge is chambered the case neck is actually crimped against the projectile. This causes tremendous pressures as the projectile tries to escape from the pinched neck. Some suppressed rifles will suffer a build up in a little as three shots, showing an increase in velocity of 100fps from the first to the third shot. Velocity increases also occur as a result of rising bore temperatures.
To check if the chamber has carbon caking, take a case that has been fired and try to pass a projectile through the case mouth. If the projectile does not pass into the case mouth easily, there is a problem. To remove carbon caking, use a harsh solvent and an oversized bronze bristle brush. In this instance, a .30 cal brush.
Other factors that affect the M70 are concentricity of the action, bedding relief points and middle screw tension. Concentricty refers to whether the bolt is square to the bore, whether the receiver threads are square and also whether the locking lugs are true to the lug rebates. Blue printing is an operation that can be adopted to remedy these problems. Bedding relief points are as described in the bedding tutorial, also outlined in the recent NZ Outdoor magazine issues. Some faces of the action need to contact the bedding, some faces should not contact the bedding. If the action feels pinched as it is pulled from the stock, chances are that the bedding job is a dud, it may have sweet spots with hand loads but will be found to be finnicky. Middle screw tension is a big deal if the action is set up in a way that if the middle action screw is over tightened, the action is bent under the force. Its best to leave the middle screw just nipped up. If the shooter is worried about it coming undone, use a bit of thread locker or even thread tape to eliminate problems.
The factors described in the above paragraph will account for around 3 MOA but not the 8 MOA you describe from your friends rifle. It is most likely a bore problem as this would account for 8-12 MOA, but without seeing the rifle I can only guess. If the rifle is accurate for the first 3 shots and you can see the link between the rate of fouling versus groups opening up, have cleaned the rifle and repeated the process and seen the same result, you know its definitely the bore. As I have said, fire lapping is the last line of action before binning the barrel.
I make my own fire lapping loads. I have my own grit and my own method of creating grit adhesion. I dont really like the simple roll on kits, most of the lapping grit falls off before the bullet is even seated in the case. I am somewhat loathe to make fire lapping bullets and send them out to your friend as mine are pretty ruthless in the pressure department. Yet I do run them at failry high pressures as a means to change the bore dimensions, loosening the bore diameter at the chamber end of the barrel with a gradual tightening towards the muzzle. Chances of the bore coming right are 50/50. Sometimes I can turn a dud into an utter tack driver as if it were a miracle. Other times nothing happens, the burr/burrs or malformed land is just too much. I dont know, maybe we can talk about this some more, how to get some fire lapping loads sorted.
The hand lapping methods I describe in the barrel break in article aren't ruthless enough for what is required here. What we are talking about is a make or break effort. It will either conform or it gets binned. The M70 is a nice rifle, more so once its customized to be honest. So don't allow your friend to get bogged down in the money side of things. If its proving expensive, walk away, have a break, borrow a rifle until a full work over can be done. With the right approach, this could be a 'rifle for life', a well balanced do it all rifle.
One thing I want to make clear guys. If you have a suppressor on your rifle and if you are able to, take the bloody thing off when doing range work. If its left on during range work, its simply unnecessary heat, wear and another variable that has to be taken into consideration. It may recoil more, too bad, learn how to control the recoil through good shooting technique. Once the test work has been done, fit the suppressor and perform a final test, checking for suppressor variables such as vertical stringing from either firing shots too quickly (heat) or carbon caking/ pressure spikes. Check for ES problems if its a long range rifle, studying how quickly you can get shots down range before the ES is ruined and a miss is likely to occur at extended ranges. After this, perform the final field zero, either over a back pack or bipod.
Lastly, optics and base mounting (loosening) can be a problem. No doubt you have already discussed this with your friend.