Volvo’s QB78 Deluxe CO2 Tanker Rifle Part 4

Been shooting the QB78 Deluxe for a few weeks now. Shooting offhand, I found that it was a bit too front heavy with the paintball tank attached--more so with the paintball tank and the Rich From Mich moderator. In an effort to lose the weight of the tank, tried both a 12g dual adapter from Umarex as well as a 90g AirSource CO2 cartridge.  The modified valve in the QB went through the CO2 in the dual 12g adapter at a record pace.  Maybe 20 good shots?  The 90g AirSource cartridge seemed a better compromise between weight/balance and a reasonable shot count.   Removed the moderator and installed an even quieter combination shroud/moderator I had on hand.  Yeah, there was some minor machining to make it fit...   This dropped and shifted the weight so it wasn't all cantilevered over the front end.  With the balance changed, now I wanted options.  I want to be able to quickly set this up for various roles.

Of course, a rifle sling was mandatory.  On a PCP or CO2 rifle, aside from making the gun easier to carry, the sling can actually be used as a shooting support.

This is a sling attachment I made a few years back to fit 88/90g AirSource tanks.  It fits over the M16x1.5 tank threads.  Can also see how the rear of the shroud is attached with set screws to the QB barrel.

Front of the shroud has 1/2-20 threads for this carbon fiber moderator.  

Picatinny rail is always convenient.   Well, it's usually convenient.  I should know better by now to not use these polymer rails.  They're OK for sling or hand stops, but marginal for bipods. A bit too flexy and twisty for me.   Had I just gone with an aluminum rail initially, I might have been tempted to call it good enough. 

But this was not what I really wanted.  

Still had some t-track rail from the QB I converted from .177 to .25 caliber.  No, it's not a UIT/Anschutz rail.  While I've used those in past projects and acknowledge that they seem to work fine, I've never been thrilled with using one as the mounting base for a bipod.  This piece of t-track from Rockler Woodworking & Hardware uses a much thicker, more substantial nut than the Anschutz rail.  Since I can mill the t-nuts, it becomes relatively easy to make a half dozen with both 4 and 5mm threads.  I can attach anything to this rail.  Anschutz, M-Lok, KeyMod, Picatinny,    Any type of sling mount, hand stop, barricade rest, forward grip, bipod, tripod...can all be made to work with this rail in a few minutes.

Here's a slice of t-track that I've already started to fit.  The only real downside is the work involved to inlet the stock.  At least I have milling cutters in the same 3/4" width as the rail.

Media blasting got the red out.

The biggest hurdle in fitting the rail was the stock's shape.  Couldn't get a grip on the tapered sides.  I tried multiple combinations of tapered shims. Nothing was particularly promising.  It's 25 degrees F in the garage right now.  Rather not damage the rifle's finish, as I won't be able to fix it for four more months.

Eventually my squirrel brain realized there was a simpler solution.  Bolted and screwed on a block of wood.  The bolt is through the existing hole for the locator stud.  The screw went into wood at the bottom of the barrel channel.  Nothing visible after reassembly.

The block gave me something parallel to grab in the milling machine vise.

With a straight edge, I leveled the forearm as best as I could.  

Centered up.  

These set ups are best efforts.  The wood stock isn't straight within 1/16". The curves aren't perfect.  If I have to do this again, I'll take the full depth in one pass.

There was a small surprise.  Notice the void in the wood.  There's still some wood putty visible.  Never saw this from the outside.  I'll fill this with superglue and fine sawdust.

The mill needs a vacuum attachment.

Of course, test fitting as I go.

The brown colored Sharpie marker was almost a perfect match to touch up the edges.

Call it camouflage in case any of the edge is visible.

There's the superglue filled void.

The screw cup got uncolored to match the accessory rail.    

This curve was another best effort.  

Marked the high spots to blend the rail into the stock.

 

Then some mounting holes. 

Countersunk.  These screw heads need to be flush--or below--for the t-nuts to slide freely in the track.  

Smoothed out the finish on the accessory rail.  

 Polished with a block of Cratex.  

Cratex is a rubberized abrasive--a big rubber eraser charged with silicon dioxide.

Wow.  All that work for this.  At least I managed the fit without mangling the factory finish.  Now I've gotta go get some Christmas decorations out.

Need to redo the rear sling mount and get some better pics up.

More in a few days.