Hi All,
I've had 2" cut off the barrel. The scores very obvious looking through the off cut.
I'll be re-tweaking the load on Sunday and have my plan of attack.
Just interested to know from those that have shortened barrels in a similar way how much they've had to adjust powder charge and was it more or less?
Just interested in personal experiences not chasing a "whats the magic figure".
Boresighting New Barrels?
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Cameron Mc
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DenisA
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Thanks Cam, it'll be interesting to see.
With 105g VLD's, my first Factory 30" 6BR barrel liked 30.4gn of AR2208.
This current barrel also ended up on 30.4gn at 32" long.
Speakng to others in the past and reading info on the net, 30.4gn seems to be a pretty common load with 6BR's despite all the different barrel lengths and profiles that are used.
With 105g VLD's, my first Factory 30" 6BR barrel liked 30.4gn of AR2208.
This current barrel also ended up on 30.4gn at 32" long.
Speakng to others in the past and reading info on the net, 30.4gn seems to be a pretty common load with 6BR's despite all the different barrel lengths and profiles that are used.
Last edited by DenisA on Sat Aug 02, 2014 12:58 pm, edited 1 time in total.
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bushtick
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williada
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Denis, if it’s any consolation I have done the same thing with a collimator but not to the same extent as you. So I looked back at your crown picture and saw something else. You have since cut off the bad section and some discussion on crowning may be relevant.
I noticed uneven carbon on one side of the crown. I also noticed your crown has no small inner bevel. The purpose of an inner bevel is to protect a modern crown. In the past a crown generally protected the bore, but a modern crown conditions the atmosphere behind the projectile for accuracy. Sharp grit, collimators or foreign objects are less likely to catch and score because they ride up on that small bevel (about 45 degrees). A full crown cut is perfect ballistically, but it is easily damaged and a small bevel prevents that and does not hurt accuracy if it is cut perpendicular to the bore and at the bore’s minimum diameter. The minimum diameter is not a worry in your case with an old barrel but I use pilots of varying size down the muzzle to find a minimum diameter and they are better than trying to measure a lead cast of the muzzle or lead slug. A new barrel blank is more likely to be belled during manufacture, particularly while lapping as the lap is drawn back or not made to size at the muzzle because of tooling limits. It is also why an inch is lopped off before the crowing to save time by some smiths.
The lathe is used for the fundamental 11 degree crown. Then nip up the inner bit at roughly 45 - 60 degrees. You can use a Brownell’s cutter/countersink which is piloted, described on another topic. When I get serious, I use the old fashioned method of grinding the crown with a small bevel using a round headed brass lap which I make, chucked in a hand drill. You can use a round headed brass screw with the same effect. Water mixed valve grinding paste is placed on the end of the bore, and the drill is used to revolve the screw head on the muzzle. As the drill chuck revolves, the upper portion of the drill is moved with a circular motion on the ball tip prevents scoring and will result in a perfectly lapped in muzzle as described by MacFarland.
It gives a perfect seal and will not yaw the projectile because like a ball cutter (a sphere) it will always cut perpendicular to the hole it is following and prevents uneven gas leakage. The lapping method also prevents the lathe bruising that delicate crown because of a blunt tool bit that I have seen on some professional jobs. Of course a truly good lathe cut with a sharp bit is excellent. But for touch up the Brownell's cutter is fine, but for major crown work of cutting back the muzzle a few millimetres will require a fundamental facing of the crown in the lathe.
Fig 1. Exaggerated view of an improperly crowned muzzle showing an abnormal pattern of the gas exit.
Fig2. Lapping a muzzle with a hand drill
Source: Harold E. MacFarland
The primary purpose of the crown in accuracy is to prevent the projectile yawing from gas leakage as per fig 1. The 11 degree crown is also thought to condition the atmosphere better than a 90 degree crown by centralising the focus of the gas. The analogy I make is that the flame on a gas torch is focussed to a point by a 12 degree copper cone for most heat efficiency. The gas from the small bevel is re-focussed by the 11 degree crown and won’t upset anything. Hope this is not over the top.
As far as your load, after lopping off a couple of inches and crowning , Raven’s suggestion is spot on.
I noticed uneven carbon on one side of the crown. I also noticed your crown has no small inner bevel. The purpose of an inner bevel is to protect a modern crown. In the past a crown generally protected the bore, but a modern crown conditions the atmosphere behind the projectile for accuracy. Sharp grit, collimators or foreign objects are less likely to catch and score because they ride up on that small bevel (about 45 degrees). A full crown cut is perfect ballistically, but it is easily damaged and a small bevel prevents that and does not hurt accuracy if it is cut perpendicular to the bore and at the bore’s minimum diameter. The minimum diameter is not a worry in your case with an old barrel but I use pilots of varying size down the muzzle to find a minimum diameter and they are better than trying to measure a lead cast of the muzzle or lead slug. A new barrel blank is more likely to be belled during manufacture, particularly while lapping as the lap is drawn back or not made to size at the muzzle because of tooling limits. It is also why an inch is lopped off before the crowing to save time by some smiths.
The lathe is used for the fundamental 11 degree crown. Then nip up the inner bit at roughly 45 - 60 degrees. You can use a Brownell’s cutter/countersink which is piloted, described on another topic. When I get serious, I use the old fashioned method of grinding the crown with a small bevel using a round headed brass lap which I make, chucked in a hand drill. You can use a round headed brass screw with the same effect. Water mixed valve grinding paste is placed on the end of the bore, and the drill is used to revolve the screw head on the muzzle. As the drill chuck revolves, the upper portion of the drill is moved with a circular motion on the ball tip prevents scoring and will result in a perfectly lapped in muzzle as described by MacFarland.
It gives a perfect seal and will not yaw the projectile because like a ball cutter (a sphere) it will always cut perpendicular to the hole it is following and prevents uneven gas leakage. The lapping method also prevents the lathe bruising that delicate crown because of a blunt tool bit that I have seen on some professional jobs. Of course a truly good lathe cut with a sharp bit is excellent. But for touch up the Brownell's cutter is fine, but for major crown work of cutting back the muzzle a few millimetres will require a fundamental facing of the crown in the lathe.
Fig 1. Exaggerated view of an improperly crowned muzzle showing an abnormal pattern of the gas exit.
Fig2. Lapping a muzzle with a hand drill
Source: Harold E. MacFarland
The primary purpose of the crown in accuracy is to prevent the projectile yawing from gas leakage as per fig 1. The 11 degree crown is also thought to condition the atmosphere better than a 90 degree crown by centralising the focus of the gas. The analogy I make is that the flame on a gas torch is focussed to a point by a 12 degree copper cone for most heat efficiency. The gas from the small bevel is re-focussed by the 11 degree crown and won’t upset anything. Hope this is not over the top.
As far as your load, after lopping off a couple of inches and crowning , Raven’s suggestion is spot on.
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DenisA
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G'day Dave, thanks for your thoughts. I had noticed the inconsistancy of the carbon star on that crown picture (leg length). Honestly, I hadn't put too much thought into it because of the more obvious problem at hand and I hadn't considered that it may not be related.
I have some barrels with the bevel in the crown and wasn't too sure of its purpose.
The carbon star on the current crown looks great.
Thanks again.
I have some barrels with the bevel in the crown and wasn't too sure of its purpose.
The carbon star on the current crown looks great.
Thanks again.
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