View Full Version : MODIFICATION to the Darrell Harrison m39 mount installation
mike in co
09-03-2005, 04:54 PM
i still like Darrell Harrison mounts, but there is a limitation on the m39 mount. the front leveling screw on the mount runs against the sheet metal that retains the rear of the upper handguard wood and the sheet metal bent in my installation, leaving me with vertical stringing. i spent a bunch of today trying to engineer a band aid for this problem. the answer is simple and easy!
my bbl is aprox 3/4" od at this point and the sheet metal is aprox 1.3" id.
i took a 5/8" gavanized flat washer, opened the id to .75", marked the od to 1.3 and made 8 cuts reducing the od to close to 1.3" then used a grinder to get to the 1.3 od. the washer was then cut in half, and one half was filed to fit under the sheet metal. a press fit and then jb welded in place. the wood handguard will reguire a minor shortening. the washer task would be a bunch easier with a lathe....but it can be done with hand tools.
the jb is drying as we speak , and i will be going back to the range tommorrow.
mike in co
09-04-2005, 12:32 PM
ok just back from the range. now i spent 9 rounds getting a new zero. then shot two loads, alternating loads on left and right targets..at 100 yds.
first four shots on both targets showed vertical stringing. went back and tightened the front and back screws more..these are now ss replacing the original brass. this caused a loss of zero and lost a couple rounds getting back on paper.
i need to load more and retest....
but the front and rear screws must be TIGHT, not snug. the short distance from the pivot pin to the front screw gives the rear screw a ton of leverage.
the good news is my last two shots were aprox .5"......
so more to follow.
09-04-2005, 08:27 PM
Please keep us posted! I've got one of those mounts I want to use on a nice M39.
Actually have two candidates, depending on how well the first one shoots.;-)
mike in co
09-05-2005, 09:43 PM
here is what i did this eve. ....i cranked on the forward screw after backing out the the rear screw, and then tried tightening the rear screw......
want some simple math. the distance from the pivot pin( rear sight pivot pin) to the front tension/adj screw is aprox .3"; the distantance to the rear screw from the pivot is aprox 3.25".
folks that gives the rear screw an 11 to 1 mechanical advantage over the front screw.
the back of the mount bottoms out on the back of the rear sight base, which in turn limits how far up the front can be adjusted with the front screw, and then the rear with its advantage just keeps turning.
here is what i did . cranked the frt screw tight, adjusted the rear to with in an degree(one degree) of level with the reciever, and then tightened the side screws.
i'm gonna re shoot, if i get vertical stringing, i'll relieve the bottom of the mount so the front screw can be turned further and try one more time.
mike in co
09-08-2005, 11:31 AM
just back from the range.
some good first indicators.
it took only two shots to obtain a 50 yd zero, and then an additional two for a 100 yd zero.
first three shots are under an inch and then
the next shot goes about an inch low....hmmmm
well the next seven go low too...so 8 shots in just over an inch.
the good news is the vertical of those 8 was less than .5"
still not sure what caused the zero move....but i'll take .5 vertical IF it will not move (again).
i'll shoot a few more getting a 200 yd zero and see how that goes, and report back.
PLEASE NOTE: my scope is set back all the way till the bolt handle just clears. if you shoot in a scope forward/scout mounting, you may not have any of these problems. also i was shooting a 174 fmjbt at 2600 fps, so if you are doing mild cast boolit loads you may also see none of these problems.
thanks and more to follow.
09-09-2005, 04:23 AM
Thanks for doing the work and reporting it. So far so good with mine; I have Burris 2x LER on it and no full house jb's. I'm a bit afraid though that the main pin right next to the front vertical screw receives all the beating and might accidentally start to shear off if too much leverage is applied to the vertical screws. I'm taking the mechanics of this mount so, that it's the side screws doing the most holding (friction) together with the pin and vertical screws are for basic adjustments; if side screws slip tiniest amount then it won't hold no matter what. I might be wrong on that, been before. I dunno if steel set screws with pointed nose would hold better on the sides, but with them some collector item would be marked permanently and fine adjustment after first one could be very difficult because of the first nose imprints. With very heavy scope hanging way back I would probably d&t inside the rear sight mount and use suitable ground block as a base and ground flat washers as a fine adjustment and use one bigger screw to tie the mount down for good. Hmm, maybe I have wrong mental picture of the rear sight innards, but I think back that there was flat area in the rear section of the sight frame. Anyway, good luck with it.
09-10-2005, 03:11 PM
Bad day at the range. I was going to zero my Marlin with Redfield 2x EER at 100 meters but it didn't happen. Redfield was changing poi something like four inches per one click and so on. I took Burris 2x from m39 and put it on Marlin, but before that I shot few 314299's. After two fouling shots and one so-so group of five I concentrated a bit better and shot five more (23.15grs. N120):
1.063" c-c; it's excellent group for me, way too good to complain, but it shows vertical stringing. I can't say it's the mount, this is easily by me or the load, but I checked the mount installation and two opposite side screws were a bit loose. I remember that I tightened them real good. This is been through only something like hundred light cast loads. Vertical screws were as tight as I care to tighten them and that means that rear screw is not tight. Well... need to keep my eye on this I think. Damn luck I needed to take the Burris off to put it on Marlin.
mike in co
09-23-2005, 10:44 PM
Ok ...maybe This Is The Last Report....but We'll See.
I Sighted In The Rifle For 200yds ( At 100)....
With My Load I Need To Be Aprox 2.5 High At 100.....
Shoot Two....1/2" But Left And Low, Crank Over And Up Shoot, And Do It Again. Now I Am About 1/4" Low, Crank Up One Click, And Fire One.......it Is Straight Above And Touching The Previous Hole....all Shots Tracked To The Scope Changes...
All This With A 4x Scope At 100yds.....
The Match Is The First Sat Of Oct........
Dang I Like This Rifle......!
mike in co
10-01-2005, 11:51 AM
ok just in from the match.
this is a mil rifle match with 4x scopes allowed , distance 200yd, from the bench.
i took four rifles(an ar10, a k31, a m39 and an ar15). the plan was to shoot the k31 and the m39 against the regular mil rifles, and shoot the ar10 against any semi 30 cal guns and the ar15 for similar rifles. we have had quite a mix of shooters and equiptment at some of these matches.
k31......199-11x( and i thought this $79 gun was shot out!!)
ar10.....197-4x( it shot right and i did not crank it to the left..lost a bunch of x's)
m39......195-5x( again shot right and i did not adjust)
i took 1st, 2nd and 4th( third was a 6.5 swede with a 196-6x)
the k31 has a darrel mount......the m39 has a darrel mount....
so if my mods are done the m39 works well. i may reshoot the m39 at 100 and see if it is still zeroed....the k31 speaks for itself...
10-01-2005, 07:10 PM
Nice shooting, Mike!:) I've taken note of your M39 mods and will try that on mine.
I've got two Burris scopes as potentials, a Handgun 4X and a Scout 2.75X. We'll see what happens.
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