RCBS Precision Mic freebore tool in ARs

[gregjc9]

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.

[Jim243]

I use the Hornady L-N-L Curved O.A.L Gauge in mine. With the bolt out.

Jim

[gregjc9]

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.

[gregjc9]

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.

[gregjc9]

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.

Precision Mic

There are 4 parts to the Precision Mic, the body, the Headspace gauge, the Freebore tool
and the Land Nut.

#1 Screw the Headspace gauge onto the body of the Precision Mic. The zero on the
body and zero on the Headspace gauge will line up. This represents Saami’s minimum
chamber dimension. Next, take a fired case (it’s good to fired the case a couple of times
so that it’s a true representation of the chamber) and place it in the body of the tool.
Screw the headspace nut down on top of the fired case until it stops. This will be where
your chamber falls in relation to Saami’s minimum. Each line above or below zero is
in thousands so you can check where your chamber is in relation to minimum to set up
your size die. Now size a case and put it into the body of the tool and screw down the
Headspace nut to check how far you are moving the shoulder. Continue to adjust your
size die down until you have the Headspace reading 1 to 2 thousands back from where
your fired case measured. Chamber the case before proceeding.

#2 The Freebore tool is the mock bullet. Adjust this tool so that the nylon connector is
approximately ¼” into the base. To obtain a consistent tension when tightening the set
screw and to help hold the tool when tightening, straighten a paper clip and insert it into
the hole in the nylon connector. Place your little finger on the paper clip and tighten the
set screw in the base of the body. When you see the paper clip start to bend, pull it out
of the nylon connector. This will be about how tight you want to tighten the set screw.
Place the Freebore tool into the gun and close the bolt. Note: The extractor must grab
the Freebore tool or you will not get a proper reading. In some cases you will need to
hand feed the freebore tool behind the extractor to get a true reading.

#3 Place the Freebore tool into the Land Nut. The Land Nut is a reference tool and
does not zero like the Headspace gauge. The easiest way to use this tool is to use the
horizontal line on the body as the reference line for the Freebore reading. As you screw
the Land Nut down, the horizontal line on the body will line up with a number on the
Land Nut. This reading is where the ogive of the bullet hits the lands in the gun. (do
this several times to be sure that you are getting a consistent reading) Example: If the
Land Nut and the horizontal line on the body of the Precision Mic line up at 40, 40 then
becomes zero for this gun. Using 40 as zero, you’ll start your first bullet seating depth at
30 (ten thousands back from where the ogive touched the lands). Load 5 at this setting,
5 at 20 (20 thousands back) and 5 at 10 (30 thousands back from the lands). When you
shoot the rounds, one bullet seating depth will be a tighter group. You can determine if
your gun likes a jump or needs to be closer to the lands. When you determine the best
group, you can plus or minus from this bullet seating depth until you find the “sweet
spot” for the gun. Note: The mock bullet on the freebore tool and the bullet that you’re
using, my not have the ogive at the exact same place but this is not important as you are
using your own bullet for testing your best group. The freebore tool gives you a starting
point.

[Unclenick]

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.

[gregjc9]

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.

Agreed. The problem I had initially from the RCBS Mic was trying to even read the measurement from the Land Nut. The land nut did not zero onto the body like the Headspace Nut does, and the instructions were less than clear. The info above from RCBS should have been in the instructions, in that the Land Nut will not zero, and to just record the measurement on the Land Nut.

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.

[Unclenick]

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.