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Thread: New .177 cal AR2078 barrel and extended probe

  1. #1
    Senior Member SeanMP's Avatar
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    New .177 cal AR2078 barrel and extended probe

    On Friday I picked up a new .177 barrel and an extended probe bolt from Eric for the AR. I would like to shoot more postal matches with the AR but most require .177 and mine is .22.

    When I first picked up the barrel I tried to shove a CP dome into the muzzle and it was so tight it wouldn't budge (Eric looked really worried but I was really pleased...of course I didn't tell Eric that)

    First step was to remove all evidence of the factory crown and as I am at home and my lathe is not I had to use my drill press set up. I basically hand tighten the breach end into the chuck and then muzzle end runs in a close fitting hole drilled thru a board. The board sits on 2" blocks and the board and blocks are clamped to the table. Then I use two equal hardwood wedges facing opposite each other so that when you slide the wedges together or apart you end up with an adjustable height block. A piece of emery is laid on my block and I move it around under the spinning barrel. (I have a picture I will edit in)

    Once the old crown was completely removed I broke the burr with a very light pass of the brass screw and diamond paste. (not worried about final crown yet.)

    Second step was to lead lap the barrel using the same process that all match barrels undergo.

    I use a cleaning jag that is meant to push cleaning felts and reduce 1/2" of the tip to about half the original size. Just below the reduced section I wind sufficient wraps of teflon tape to get a snug fit. This assembly is pulled into the muzzle until the tip of the jag is an 1/8" below flush. The rest is simply pouring melted lead down the bore until its over full and allowing it to cool. But .177 is so small it's impossible to get lead to flow down the bore without it solidifying instantly so the tip of the muzzle has to be heated to above the melting point of lead. Just above...don't go crazy.

    Once it cools it knocks out easily and the sprue gets trimmed with a sharp exacto knife

    Lead lap.jpg

    Once the lap is trimmed and ready it needs a shallow groove cut into it to hold the lapping compound.

    I only use JB Non Embedding Bore Compound and I don't recommend anything else. It's available from Brownell's and several sub distributors in Canada.

    Pass a felt/patch that has been soaked in light oil thru the bore then coat the lead lap in JB compound. Working from the breach wiggle the lap around until the rifling lines up and start taking full length passes all the way to the muzzle. Let the lap protrude out of the muzzle a bit but don't allow it to pop right out. The first few passes can be a bit snarly but it will quickly get smoother.

    Continue taking full length passes until the bore feels even along the full length (25-50 passes) adding more lapping compound every so often. If the lap feels loose before you are happy with the feel of the bore simply tap the end of the lap once lightly to swell the lead.

    Once your happy with the overall feel of the bore mark the rod 1" before the lap hits the muzzle then take three passes at this length. Then mark 1' shorter and three more passes and so on until you work your way backwards in 1" steps all the way to the breach. Tap the lap on the end to swell the lead as necessary. This is the technique used to lap a choke.

    Final take several dozen passes at the breach throat.

    At this point I completely clean the bore with oil soaked patches or felt then I use a patch that has been saturated in JB Bore Bright to polish the bore using full length strokes but again not allowing the patch to pop out completely.

    Completely clean the bore again using oil soaked patches/felts followed by dry until the bore is completly clean. (you'll notice the patches just glide thru now.)

    Once this was all done I finalized the crown using the brass screw technique, pushed a felt thru and reassembled the rifle.
    crown.jpg

    The assembled rifle with some adjustable brass barrel weights I use for tuning

    adjustable barrel weights.jpgBarrel.jpg

    The new extended probe bolt installed

    extended probe.jpg


    Here comes the exciting part

    First 5 shot group with the new barrel.

    Meisterkugelns at 10m

    Standing with forearm rest

    Using factory target sights


    Are ya ready for this.................


    first 5 group.jpg

    It's a one hole wonder

    The blob of lead above the hole is the 5 pellets welded together that I dug out of my putty backstop.

    With some tuning as far as pellets, velocity and barrel wieght I'm sure the accuracy will only get better.

    The only downside is it appears that I am going to have to do some shimming at the breach because I ran out of sight adjustment before I got a zero

    But I'm flippin thrilled

    shhhh don't tell Eric
    Sean

  2. #2
    Administrator AirGunEric's Avatar
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    Your one hole is still 2" off target.
    I'd say I care- but I'd probably be lying...


  3. #3
    Senior Member SeanMP's Avatar
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    Quote Originally Posted by AirGunEric View Post
    Your one hole is still 2" off target.
    Ya it is and I blame you ....don't know how or why but it's your fault
    Sean

  4. #4
    Administrator AirGunEric's Avatar
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    Yeah, I'm married- it's always my fault...

    I'd say I care- but I'd probably be lying...


  5. #5
    Senior Member SeanMP's Avatar
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    Lol...ditto

    Well I got it to zero but I had to remove the rear riser block under the sight. I have checked everything over and I'm mystified, everything is straight and properly positioned. It just likes to shoot high.

    Having said that even with the old barrel the rear sight was almost at the bottom of the rack.

    The gun is shooting well enough that it really deserves something like a Centra front sight tunnel then it could be raised up using riser blocks. But I would have to have a bloob tube made with a dovetail on it to mount the Centra
    Sean

  6. #6
    Member Gippeto's Avatar
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    Interesting and educational write up Sean... with bloody excellent results.

    Thanks

    Al

    Edited for shpelung

  7. #7
    Member Dukemeister's Avatar
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    Amazing Sean! Well done.
    Member, Limestone Beekeepers Guild
    Duke ))))----//----------==

  8. #8
    Moderator rsterne's Avatar
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    Excellent writeup on lapping a barrel.... gonna have to try that at some point.... Thanks !!!

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal:
    .177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26, DAQ .308 Exile

  9. #9
    Senior Member SeanMP's Avatar
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    Thanks folks. I really enjoy barrel work and always have.

    All is not perfect in paradise yet. I am fighting a small leading problem. As I put rounds through it the groups are slowly opening to about .200". I pull the barrel and go at it with Bore Bright the groups go back to tiny. And so on all day.

    I think I found the culprit though. There is a tooling mark 1mm inside of the crown and I can see some streaking on it.
    Sean

  10. #10
    Senior Member SeanMP's Avatar
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    Quote Originally Posted by AirGunEric View Post
    Your one hole is still 2" off target.
    A quick update just for EricIMG042.jpg

    I'm not happy with your stinking barrel at all
    Sean

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