Friday, February 19, 2010

Front Sight Adapter for the QB77

After getting the trigger shoe fit to the QB77, I turned my attention to the sights.

The QB77 is extremely accurate but is hampered by the open sights. I've already got a similar rifle with a scope--a Crosman 2260--so the aperture sights provide some variety. I found this vintage Crosman 411 sight in the basement and thought it needed some gun time.

The rear sight mounted right up. The Crosman 411 front sight was a no-go from the start and didn't look like it would be easy to retrofit.

I had a spare Weihrauch front sight but it requires a dovetail on the barrel to fit. An adapter is needed. Looking at the rear sight again, I found that in the middle of the elevation adjustment, the sight was about 0.710" from the top of the breech. To get the front sight to work, it's center should be close to that number.

Found a scrap of rectangular aluminum in the pile for the adapter. The piece was 0.382" x 0.510" and about 1.55" long.

The widest point of the HW front sight dovetail is about 9 mm wide and has 60 degree angles.

Mounted the block in the Taig milling attachment on the lathe. Milled the ends off square.

A dab of dykem and found the center. Worked off the center line and scratched some lines on the top that reflected the 9mm width of the dovetail.

A small 60 degree dovetail cutter and lots of Tap Magic aluminum cutting fluid. Cut just shy of the lines, then test fit the sight. Another couple thousandths and it was on the money.

Flipped the adapter over and used a ball mill to cut away excess material. If I had the correct size ball mill (14 mm), I'd be done--but that'd be too easy.

Went through the transfer punches and found a punch just smaller than the barrel diameter. Wrapped it in emery cloth and hand-sanded the contour to size.

Like so.

Another view.

Just need to drill a hole to attach the adapter to the front sight screw hole. Drilling 1.104" from the muzzle. Used the Taig as a horizontal drill press. Drilled with a #32 bit to clear the M3 x 0.5mm sight screw.

Then counter bored the hole for the head of the fastener.

Done except for some clean-up sanding and some color.

Dykem comes off easiest with their remover. Not shown: Took all the sides of the adapter to 320 grit on a granite plate. Used Birchwood Casey Aluminum Black to color.

Installed. Left the M3 screw silver. It won't show after the globe sight is installed.



More views.

Weihrauch sight installed with one of several inserts.



Sights installed. No permanent alterations. The original sights can be reinstalled in about 2 minutes should that ever be required. The QB now shoots like a laser beam.

1 comment:

jimziegler said...

Derrick and Nick. this blog is awesome. great info with amazing illustrations