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> Precision long range hunting and shooting
> Double Grouping
Double Grouping
@ 10:17 am (GMT) |
Chris BrooksHi NathanI've bought your two books and found them extremely interesting and informative. In the course of reading them, I think I have found an answer to a long term rifle problem. I have a Tikka M690 in 30-06 professionally bedded in to a McMillan stock. The rifle is generally unfussy about ammunition and will put the first two shots into .4" the next 2 shots will group a similar size but about 2" away at 5 o'clock. A fifth shot will land anywhere between the two groups. The bedding is devcon and is an extremely tight fit which you suggest might be the cause. I have checked all the mounts and the bedding screws are torqued to 55in/lbs. Changing the scope makes no difference. Is there any way that I can correct this without completely rebedding the rifle? Many thanks Chris |
Replies
@ 03:49 pm (GMT) |
Nathan FosterRe: Double GroupingHi Chris, I am a bit worried that your rifle has a heat treatment flaw in the barrel. All we can do now is go through the steps.There is a high chance that the smith did not properly locate the lug during bedding. Actually I just about guarantee it. The boss on the rifle action is most likely located with the center of the hole on the ali block when it needs to be located so that the rear of the action boss locates in the hole. We cannot change this now without a full rebed but we need to know whether it is the barrel or bedding. Step 1. Relieve the side walls of the bedding. Take a fresh peice of sand paper. About 180 grit. Sand one side and count each pass. Do 6 passes (12 back and forth). Change to fresh sand paper and do the other side and count your 6 passes again. Lube the side walls, then check fit. If the fit is still tight, keep going. Alternatively, strafe with glass bead blaster (need to tape / protect bottom face of bedding). The fit needs to be loose enough that the rifle can recoil back against the lug. 2. grease the metal work and re-assemble. The grease will act as a protective coating but in this instance also helps the rifle recoil to battery. Torque action screws no more than 45 inch-lb. 3. Observe group behavior. Fire one group at an even pace. Fire one group at ambient. By ambient, I really mean letting the barrel cool right off between shots to within a degree of ambient temperature. Bear in mind- you could perform this test to begin with. A group like this can take about 15 minutes to shoot. You need something to do (like read a book) so that you don't rush it. 4. If the bedding relief worked- great. If not, the job is a bust. Pull the barrel, grind out the bedding and start over. Obtain a new lug from Midway etc. |
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@ 07:41 pm (GMT) |
Chris BrooksRe: Double GroupingHi NathanThank you for the advice. I relieved the side walls of the bedding as you suggested until the action was a nice smooth fit with no binding, taking care to sand equally on both sides of the bedding with fresh sand paper each side. I also measured with a vernier, the round boss that fits the bedded recoil lug. It is a precision fit, so I guess the smith must have made it rather than buying one from midway. Anyway I greased the action, put it back in the stock and torqued the action screws to 40 inch-lb. I headed up the range to day and after a fouling shot, I then shot a slow 5 round group allowing 5 mins between shots. Result - ragged one hole group measuring .3" I then shot another couple of 5 shot groups at a normal pace. Both came in at .5" I am extremely happy and relieved to say the least!! Thank you for your help and advice. I reckon your books are the best £40 I have spent in a long time. |
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@ 03:34 pm (GMT) |
Nathan FosterRe: Double GroupingThat is great news Chris. Theory is one thing but having practical info that can be put to use is key. It is really good to have test cases like this on the forums here. Others will be able to benefit from having seen you work through this problem and achieve a successful result.My next book is a full on how to book with numbered steps regarding accurizing and problem solving, same as we just went through here. But I needed to outlay the fundamentals in the first book, otherwise (as an example), you may have been a bit nervous about performing the above tasks. Instead, you had an understanding of what was required of the platform and got straight down to achieving that goal. Excellent. |
@ 08:42 pm (GMT) |
Martin TaylorRe: Double GroupingAdvice like that is very hard to find, great pick up Nathan, clear, straight foward instructions that work.A big win for you Chris! Well done, armed with the knowledge in Nathans books you won't need to pay the "professionals" next time you need a bedding job done. |
@ 06:52 pm (GMT) |
mark whiteleyRe: Double Groupinggoes to show the drop and plop method is not the way, if you want the best.regards mark |
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