Old Skool Archer Cothran Furr QB78

[mackeral5]

When I first started the hobby it was with QB78's.  This quickly escalated to QB78's with everything Archer sold for them, even some of his gimmicks such as a little horseshoe shaped piece of metal that was marketed as a debounce device.  it may have very well functioned as designed, but i managed to destroy it during my first attempt at reassembling a cock on closing QB78.  Also included were numerous of Archer's Repeater kits, which I believe he ultimately sold to Umarex, which in turn gave us the Gauntlet....

The escalation continued to many different contraptions with bottles hanging under them, to the side, and whatever else I could come up with.  Cothran double tubes, SSG's, light hammers, bulk fill CO2, a bulk fill/4 12g cartridge tube from TRRobb in the UK,  low pressure HPA, high pressure HPA....All the way up to experimenting with a 36" tubed, Cothran powerhouse-valved .357 at 250-300fpe. 

The gun pictured here was sort of the sum output of all of that.  It is a QB78 with one of Archer's repeater kits.  It is probably the better QB barrel from my stash.  It has a 25" SS tube made by Don Cothran and quieting things down is a moderator from GTA's very own David Furr (Rocker1.)  Inside you have inspirations from Bob Stern in the form of an SSG (which is equipped with a flat wire spring, inspired by Don/Nvreloader).  Bob's grouse gun build, IIRC, is what made me realize we didn't always have to make the valve bigger to get more airflow, hence converting to a 2mm stem is one of the early things I do to any gun needing more throat area.  I collected many of the breadcrumbs dropped by Scott/Motorhead over the years and they are present in the form of an ultralight peek hammer opening peek poppeted, air spring/cobra chamber equipped valve.  Robert Dean/Ribbonstone contributed greatly in the form of helping me realize that parts didn't always have to be pretty to function properly....with most parts, especially internals, there are key dimensions and characteristics, but a lot is up for modification/variation or even being left unfinished. 

Not the actual hammer/SSG from this gun, but representative.  I no longer make my SSG's externally adjustable as I prefer to not have thing hanging out the back if I can avoid it.







All of these ingredients resulted in a typical tune of 30 shots of JSB 18.13gr at 890fps.  Not too shabby for a fairly small amount of air storage. 

This particular gun has been hidden in the back of the safe for quite some time now.  It always had a slight leak that I never felt like addressing.   

Today I pulled it out and decided to freshen it up a bit and solve the leak.  Putting a balloon over the muzzle verified a leaky poppet.  I freshened up the valve seat as well as poppet face and now she seems to be leak free. 

The tune is still basically the same, ~30 shots of JSB 18.13's around 895fps. 

it still shoots.

Here is a 50 yard target. 



Even though the wind is swirling I did try my luck at 100 yards.  I was able to sneak 1 group, (the group circled) at just the right timing with regards to wind.



Recently I have been spending a lot of time shooting guns tuned for >100fpe....this was a nice reminder that guns do not have to consume large quantities of lead and air to be fun, accurate. 

This was also a reminder that a 32fpe .22 is nothing to sneeze at, it is pretty capable for a "pellet gun."

It is long.  It is fairly heavy.  It does not have a sidelever nor is it semi automatic.  But, like many old things, it still works and performs at a decent level.

The tree rats caused a bit of destruction this winter, I think this will be just the tool to re-establish boundaries and do a little thinning of the herd. 

[stroo]

Cool build, thanks for posting!

[Goose]

That is a nice build!

I wasn't aware of all that was available to bolt on to these guns until you posted about yours, but I've been a fan of the system since Tim introduced me to the first QB77s to enter the country.  We even had one made up in 5mm with a L/W barrel.  Glad to see you giving some of the glory to Ribbonstone.  His posts taught me a bunch.

They're the 22R of the airgun world.  (Tim's a fan of those as well.)

J~

[Gippeto]

Good stuff Mike, QB platform is likely my favorite to tinker on. Re-working my QB79 currently and started back in on a Qb78 project today as well.

I appreciate not having things stick out the back as well.

[renowntwo]

Was going to make a post about the about my  2020 qb.

[mackeral5]

Time for an update on this one.  In recent weeks i found myself needing to refocus on the hobby and purge distractions.  This meant sending several project guns and parts on to new owners so they can continue their journeys and be used vs sitting and collecting dust here.  Plus, as long as project guns and parts are here, there is risk they will be the next "squirrel!" I see and start down the path of some unplanned project, just because the parts were in my line of sight. 

After about maybe 2 dozen or so shipments I finally got things whittled down where I could become a bit more focused.  i stumbled across a random .22 FX liner I had laying around. it's an odd length, 480mm and I do not recall what gun it came out of.  I almost decided to put it in an old BAM B51 needing a barrel, but stopped short when I started taking inventory of other available parts.  The BAM project will hopefully find a new owner before I find a barrel for it. 

I had an old QB barrel, modified for use with the repeater breeches but it never was a good shooter.  I also found a piece of 8mm x 13mm CF as well as a 13x15mm piece.  While I am not planning to build anymore carbon fiber barrels, I figured it would not hurt to do one more, this time for the "almost perfect" (in my eyes at least) QB78 that is the subject of this thread. 

I cut off 1.75" of the breech section of the old QB barrel and using my drill press and cross slide, crudely carved it into a breech adapter, or thimble or whatever we want to call it. 

here are the pieces.



After sanding the ID of the 8mmx13mm tubing to allow a slip fit of the liner, I assembled everything using loctite 680.  After carefully cleaning things up and coating the breech end with hornady case lube, I put the assembly in the QB78 and let it sit in the sun for a few hours allowing the 680 to cure. The hornady case lube was there to prevent the barrel and breech from becoming one due to any loctite 680 leaking.  A lesson learned the hard way on previous projects...

I guess I was too excited to get things done, so I do not have any pics of the assembled barrel.  Here is the assembled gun. 



As pictured, it now weighs in at 7lb 3oz.  I do not have the previous weight handy, but I am fairly certain she lost at least 1lb.  Overall length is 43" with the Tanto mounted.

One more step taken, but still not quite "perfect".  The next mod will be to add a detent to the bolt/breech, to provide positive bolt closure and prevent the bolt from being turned out of battery so easily.  I am uncertain when this will happen, it will likely be some time before I am interested in taking it back apart.

Some time back a regulator body oring let loose and I had it apart for resealing.  At that time I turned the reg down from 117 bar to 110 bar.  Currently it is tuned to send JSB 18.13 @ 835fps for 28fpe.  Next time it is apart I will likely dial it down to 100bar. 

Here is a 50 yard target shot from a 3000psi fill.  Each bull received a single 10 shot magazine, shot fairly rapid fire.  Group 1 was top left bull: group 2 top right; group 3 lower left; group 4 lower right.  I was pretty sure it was well off reg by now, but just to be certain I shot those 3 shots below the center bull.  I shot one last shot over the chronograph, shot #44 was down to 685fps....  Ending pressure was 1300psi. 

 

[Goose]

Nice work, Mike! 

It's really good to see you keeping the QB78 love going.  They fall right into my sweet spot of affordability and tuneable accuracy.

Where do you source your barrels?  I've been looking for a .177 bolt and barrel for my .22 and haven't had any luck.

Stay safe,

J~

[mackeral5]

Nice work, Mike! 

It's really good to see you keeping the QB78 love going.  They fall right into my sweet spot of affordability and tuneable accuracy.

Where do you source your barrels?  I've been looking for a .177 bolt and barrel for my .22 and haven't had any luck.

Stay safe,

J~

Thanks Jeff...It is the only one I have left and I have offloaded most of my spares to ensure I do not build any more...

IMO, The best way to get QB spares today is the same it has always been---buy another QB for $100 and part it out.  You can sell the valve for $25, stock for $20-30,  tube for $20?  trigger assembly for $20?  etc.....In the end you should be able to easily sell enough parts to only have $25-50 in your caliber swap kit.

[ranchibi]

Very nice work on an awesome build and boy can it shoot! Nice write up and pics.

[Racing]

Just love it Mike, and sure rings a bell as far as my latest Chief project.

[IMADMAN]

Nice build Mike!
I have a QB built by @@$$$&^(, .30 cal. Extremely fun gun, if you just want to sit, take your time and plink. Shooting round balls out of it.

[mackeral5]

 I appreciate the kind words.  if I let myself think about it, the bits and pieces of this gun have seen a decade of life's seasons, the changes, some were good, some were something other than good. 

Strange to see a builder's name blocked out like that, I thought those days were behind us? 

I shot one last 10 shot group before calling it done for now.  I am now convinced the FX liner can outshoot the OEM QB barrel(s) that preceded it.  The question is, will it last???  Only time will tell.....

[ranchibi]

Fantastic groups! I'm pretty sure that liner will stand the test of time since you made it more rigid and sturdy with that carbon fiber liner 👍.

[mackeral5]

Thanks Randall.  So far so good.  A couple changes since last update.

Here is a current pic. 



A close look will reveal something I believe never photographed with this gun... The safety is installed.  Any serious QB modder knows this means for all intents and purposes, the tinkering phase is over.  Some modify their stocks to allow the safety to stay with the action when removed from the stock, I am not a good woodworker so I never risked further hacking up the stock... 

Also of note, I replaced the DonnyFl Tanto and steel slip on 1/2-20 adapter with a lightweight reflex style unit designed by Subscriber and printed by Torquemaster.  This took a couple of ounces off the muzzle and maintaining very close to the same overall length.  The 3d printed cores are housed in a piece of 26mm ID 1mm wall carbon tubing I found in my scrap bin.  Sound is very similar to the DonnyFl Tanto, perhaps even slightly more quiet. 

Accuracy seems to be the same or better than before, as evidenced by these 2 rapid fire 10 shot groups at 50 yards.  I changed POA slightly higher on the second group, trying to move it to the bull but I guess I over-corrected and shot over the bull.