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Air Gun Home Forum Index » Airgun Smithing » New .177 cal AR2078 (QB 78 Comp) barrel and extended probe
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New .177 cal AR2078 (QB 78 Comp) barrel and extended probe 
PostPosted: Thu Feb 09, 2012 8:48 am Reply with quote
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Joined: 04 Feb 2011
Posts: 1183
Location: Just south of "Out There"- Land O Lakes Ontario
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) Laughing

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

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.

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

The new extended probe bolt installed

Here comes the exciting part Pink Bouncer

First 5 shot group with the new barrel.

Meisterkugelns at 10m

Standing with forearm rest

Using factory target sights

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

Pink Bouncer
Bouncing Smoker
Pink Bouncer
Bouncing Smoker

It's a one hole wonder Mr. Green

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 WooHoo!

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New .177 cal AR2078 (QB 78 Comp) barrel and extended probe 
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