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January 10, 2011, 07:56 PM | #1 |
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RCBS Precision Mic freebore tool in ARs
Anyone use the Precision Mic freebore tool in an AR? Easing the bolt closed on mine, the tool wont get engaged by the extractor, so I have to use the cleaning rod to push it out. I've got the set screw about as tight as I dare to get it.
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January 10, 2011, 08:08 PM | #2 |
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I use the Hornady L-N-L Curved O.A.L Gauge in mine. With the bolt out.
Jim
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January 10, 2011, 08:41 PM | #3 |
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Ok, I think I got it. I tightened it quite a bit, and worked it under the extractor, and eased the bolt forward, using the forward assist to seat it.
I got consistent measurements, but I'm a little unclear on how to read it. I know the marks on the body are .050" and the marks on the top are .001", but the 0 marks dont line up - but the 25 mark on the top lines up with the 0 mark on the body (actually the 20 mark looks to line up more precisely). So I'm not sure exactly how to read what I got. Using the 20 mark on the land nut as the 0, I read .081". I know how to read a mic, but I really dont think the land nut is machined correctly. The headspace nut lines up like it should, both on 0. I've attached a couple of pics. TIA for any assist.
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ΜΟΛΩΝ ΛΑΒΕ Greg Last edited by gregjc9; January 10, 2011 at 09:02 PM. |
January 11, 2011, 12:23 PM | #4 |
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Just got off the phone with RCBS, seems the Land Nut wasnt calibrated before the set was shipped out. I'll be sending it back to them this week for a replacement.
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January 14, 2011, 11:40 AM | #5 | |
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Earlier this week I had sent in a question to RCBS regarding reading the measurement of the Land Nut. Below is their response, hopefully it will help others. It makes sense, but I wished their instructions were clearer. Bottom line, only use the horizontal mark on the body as a reference line against the Land Nut.
Now I have to wait for Midway to send me a replacement, as RCBS' wonderful CS department told me it wasnt calibrated and I need to send it in. Quote:
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ΜΟΛΩΝ ΛΑΒΕ Greg Last edited by gregjc9; January 14, 2011 at 11:46 AM. |
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January 14, 2011, 01:47 PM | #6 |
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Greg,
Land position is all about relative measurements, as different chamber reamers (.223 Commercial, .223 Match, 5.56×55) have different freebore lengths. Indeed, as you shoot the gun and erode the bore, that number gradually gets longer. All you need to care about is writing down whatever measurement the thimble comes up with when the freebore tool's "bullet" is contacting the lands. You can then control the seating depth for your single-feed slowfire bullets below that point. You'll discover each bullet shape you fire has a different optimal seating depth. Retake the freebore reading every couple hundred rounds to watch for throat growth. BTW, I own a couple of PM's that I got before I owned the Stoney Point (now Hornady LNL) style gauges. I always had trouble trusting the freebore tool. If I tightened the screw enough, the "bullet" would tend to stick in the throat and pull out upon extraction. Too loose, and the bullet also pulled. Especially in a worn throat that had a very shallow angle. I finally found a sort of middle ground. I would strip the extractor and ejector off the bolts and used the bolt body just to close on the tool. I then opened the bolt and used a cleaning rod to gently push the tool out backward so the "bullet" position wouldn't shift. Much bother, but more precise.
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January 14, 2011, 05:59 PM | #7 | |
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Quote:
In addition to taking the freebore measurement every couple hundred rounds, wouldnt it also need to be taken any time one is loading different bullet mfgr/makes? In loading pistols I've noticed a fairly signifciant difference in bullet ogive between different mfgrs, and assume same would be for rifle bullets. Thanks again for the info.
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January 14, 2011, 10:37 PM | #8 |
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Yes, they should have made it clear that there is a SAAMI minimum chamber for the headspace, but because every chamber is different there is no standard for the throat location.
Every bullet will have a preferred distance off the land and the only contact point will be where the ogive departs from the bearing surface. The tip of the nose will indeed be different distances from that contact point for different bullet designs, and so will the different designs prefer to be seated different distances from the throat for best accuracy. In general, the longer the ogive the further from the lands it will want to be. Berger says some VLD's in some rifles have to be as much as 0.165" off the lands for best accuracy. Some more blunt shapes, like the 77 grain Sierra MatchKing are often happier more like 20-40 thousandths off the lands. You just have to find that distance for each bullet for your chamber, then adjust it outward when you discover the throat has moved forward. None of the long bullets and usually none of the stubbier ones will be at best accuracy when seated short enough to fit in a magazine. The whole technique of using the PM or other gauge to adjusting how far you seat bullets off the lands tends to produce rounds too long to fit in a magazine. These are going to be for singly-loaded rounds for the slow fire prone stage of competition, where the target has the smallest moa scoring rings and the 600 yard range demands the best ballistic coefficient bullet your cartridge can use efficiently to help buck the wind. In the .223 this is usually 80 grain bullets. Some folks use the 90's, but the loss in velocity from going that heavy with the .223's powder capacity often proves too much for the additional BC to make up for. I have an illustration somewhere of two different shape bullets seated the same distance off the lands, which means they would read the same on the PM after being seated, but which have nowhere near the same COL. We need a new term for ogive length from the head. OLFH doesn't exactly roll off the tongue. Maybe Ogive Overall Length would work? OOL. Rhymes with "cool". Better than Determined Relocated Ogive Overall Length. Later: Found the illustration.
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