Thursday, March 4, 2010

Aperture Retrofit for the QB77

The Crosman 411 aperture sight was a nice addition to the QB77, but I wanted a larger diameter disc. The 411 disc is also too small in diameter to fit a rubber eye cup.

I was ready to make a new disc when I remembered that I had a spare aperture for a 10-meter match rifle.

I think it's an old Anschutz.

Size comparison. The Anschutz (?) has a weird M9.5 x 1mm thread. Then again, the Crosman 411 has a weird #12-28 thread...

After looking at the aperture for a while and making some comparisons to the 411 sight base, it looked like a fairly easy retrofit. Chucked the aperture into the 3-jaw to begin.

The ID of the threaded tube was exactly the right size to tap 1/4-28. Started with a plug type tap.

Followed with a bottoming tap.

Finished by hand with a proper tap handle.

A scrap of 0.250" drill rod about an inch long.

Faced both ends.

Turned one end down to approx. 0.212" or thereabouts.

Flipped the rod in the jaws and threaded the larger .250" diameter to 1/4-28.

Turned the rod around again and tapped the smaller diameter to #12-28.


And through drilled with a #29 bit.

Cut a small relief at the base of the #12-28 thread.

Scrounged a washer/bushing/spacer from a parts bin.

And bored out the ID to just fit over the exterior threads on the aperture.

It's strictly a cosmetic touch to cover the unused exterior threads.

Removed the elevation and windage plate from the sight base to test fit.

Greased the double threaded adapter and snugged it down into the aperture. Pulled the rubber eye cup off the HW 35E.

Anschutz (?) installed.

Looks exactly like what I was after.

Since I didn't damage or modify the exterior threads, the Anschutz disc can be still used on the correct base as well.

I'd still like to refinish the stock, get rid of the knurled nut with a recessed fitting , add a rear sling swivel stud, overhaul the rifle... All in good time.


Ryan said...

Excellent! I've been wanting a Mendoza aperture sight for a while now. Don't know what gun I'll put it on if/when I do get it.

Ok, the above is my QB78 all back together. I've grown to like the nasty knurled nut on the bottom, though I think I'll recess it a bit.

Two questions: 1) as you can see in the picture, I've converted my QB78 to bulk C02, and am currently using AirSource. What do you think of the idea of slapping on a 13CU high pressure air tank, regulated to 900psi output? makes one with both in and out burst disks.

2) I've "floated" my barrel, by simply removing the barrel ring. The allen bolt that holds the barrel to the breech seems plenty secure to me. What's your thought on floating the barrel this way?

Thanks for the fun blog!

derrick38 said...

Wish I could get used to the knurled nut. It's exactly where I want to rest the gun on my offhand. Almost as if they knew...

If you really want to go to air you probably already know what you need to know. But if not, aside from the added expense of needing all the PCP fill equipment for the 13CU tank (maybe you already have all that) what will you gain? Your shot count is going to drop, you'll have to refill more often, and your pellet velocity will be the same. The only positives I see are no temperature sensitivity if you shoot in cold temps and cheaper refills--though that might take years of usage to achieve if you have to buy the tank plus all the fill equipment.

I don't think the breech alone offers enough support on a QB action to adequately support the full-length barrel. Also, I haven't noticed poor grouping due to the barrel band, so I'd reinstall it. There are nicer bands out there if you don't care for the plastic.

Ryan said...


All good points. Regarding, fill equipment, the place that fills my C02 also fills HPA.

You mentioned one of my hoped-for benefits: less temp sensitivity. Though truthfully, I rarely shoot outdoors.

The bigger desire I have, is to address muzzle heaviness. With a AirSource tank on the front, the rifle is pretty barrel heavy, with a 9oz CO2 bottle it's ridiculous.

I haven't actually held a 13CU HPA tank, so I don't know what it weighs. I'm guessing the same full as empty though. :) My hope is/was that it would be a lot lighter than a 9oz CO2 tank full of... well 9oz of CO2.

And you called it. I hate the plastic barrel band. I wonder, would a 4 or 5" long aluminum shim between the barrel and compression tube would be sufficient support.

For now, I think I'll take your advice and reinstall the barrel band.

Finally, shot count. I was figuring, if a full 13CU tank gives about 125 shots on a 9mm marker, I should get around 300 shots on the QB78. The QB78 won't use any air for semi-automatic action, and it's bore diameter is only %61 of a paintball marker. 300 regulated shots per fill is fine for me, I think. But is it worth the $60 investment? I'm still deciding.

Thanks for your very cogent response Derrick.

derrick38 said...

I'm not sure how bore size relates to figuring out shot count, but the valves in PB guns have pretty small volume to keep the velocity low enough to not hurt players. Might try the Crosman Forum and ask about shot count. I hear you about the weight, but I prefer a muzzle heavy rifle and 10 to 12 pounds is the norm for 10 meter match guns. Just don't ask me to carry one in the field. That's why we need rifle-bearers.

Ryan said...


I was trying to wring out a simple equation to make my shot count comparison/prediction.

The valves of both paintball and pellet guns are tuned to move an average projectile mass a pre-set speed. In the case of the paintball marker, a larger mass with a bigger volume barrel to pressurize in order to propel that mass.

Conversely, the QB is setup to propel the smaller mass of a pellet with the smaller volume barrel to pressurize.

Bigger paintball mass + larger volume barrel = more air dumped. Smaller pellet + smaller volume barrel = less air dumped. I guess the part I left out of my equation was the higher velocity of the pellet gun. The average muzzle velocity of a paintball marker is 300fps, the QB78 shoots around 600 fps.

It might be the case that it all just evens out and both the marker and the pellet gun would get about 125 shots. I haven't even tested how many shots I get currently with a 9oz CO2 tank on the gun, so I have no real idea of that baseline.

I'm sure a comparative experiment would produce better information than my goofy math. If I get antsy to dump $60, we may find out - though you've pretty much talked me out of buying the HPA tank and reg. :)

derrick38 said...

Ryan, I don't want to talk you out of anything. If it's only $60 you can probably recoup that money if it doesn't work out for you by reselling it on the Yellow Forum classifieds. Now, if you had to buy a SCUBA tank...that's quite another cost factor.

Not sure how much of this applies to your situation: