SpudFiles

I've been thinking (uh oh) again about barrels.

First, rifled chokes. I have a scrap .22 cal that maybe could be used as a rifled choke on the end of a smoothbore barrel. There is a hypothesis that a high speed slug slamming into the rifling might be bad for accuracy, but at pneumatic velocities probably a non issue.

Then the thought occurred to me: hop up starts at the breech. This prevents random spins down the barrel. So, if I put my rifled section at the breech then the remaining smoothbore barrel functions for velocity and perhaps even smooths off the engraving on the projectile.

Has anyone tried such a thing? I live where rifled barrels can be expensive, especially long ones of large cal, so any way to save would be awesome.

Another thought: conical, or squeeze bores. I found some tapered stainless tube... start the projectile with extra force, squeeze it down for better sectional density. JSR links in 3,2,1...

I was thinking a stryrofoam pusher... what are the advantages over a discarding sabot anyway?

Anatine Duo wrote: ↑

Wed Aug 18, 2021 6:45 am

JSR links in 3,2,1...

How dare you assume exactly what I was going to do

There are precedents for both ideas.

Apart from rifled shotgun chokes, having a small rifled sectioned at the muzzle is also the principle behind the FX smooth twist barrel that by all accounts works very well:



As to rifling at the breech, these WWI air mortars had less than a projectile length of rifling that studs on the projectile fitted into and it seems to have worked well.

Also, look at the Paris Gun:



The last 6 meters of barrel are a smoothbore extension, unlike what is claimed in the wiki article from what I remember in Gerald Bull's book, this was to boost velocity without the complexity of having the barreled sections line up.

Of course you might have some air loss at the transition point depending on how it's done but the next benefit should be positive.

I live where rifled barrels can be expensive

You and me both, I have several firearm barrels that I intend to use for pneumatic projects and I'm similarly keen to cut costs.

conical, or squeeze bores

The "Gerlich Principle" as used by the 2.8 cm sPzB 41 and its bigger caliber offshoots works really well for powder burners but it's not really worth the trouble for pneumatics. The whole point is to sustain pressure in the barrel as the projectile travels down it, you can get the same effect in a pneumatic by just using a larger chamber or longer valve dwell.

Also worth mentioning to strengthen the case for a rifled breech section with mostly smoothbore barrel, the squeezebore Pak 41 as well as the guns that used the Littlejohn Adapter both used smoothbore tapered sections.

So interesting! I was also considering modifying a current coax muzzleloader to have a longer barrel... if successful there is huge potential for adding rifling or choke or simply barrel length.

One thing to keep in mind is that the rate of spin is going to depend on projectile velocity, so while a rifled breech might be attractive because it's more gentle on the projectile, a rifled muzzle is going to interact with it when it's at its fastest.

Question on barrel rifling expense, is a rifling button with budget? For .22 there are plenty of carbide rifling buttons for sale on ebay around $50. I haven't used one myself but from what I've seen if you rifle quality steel with a cheap rifling button you'll want drill rod and a press to force it through.
You might have an easier time with a trying that rifled breech only idea or the a softer metal since it's only for a pneumatic

Ever considered ECM rifling? I believe the FGC-9 and it's ecm'd barrel have been shared here before;



Smith and Wesson has been using ECM to rifle barrels for a while now....a quick look at the commercial version of the process;



Looks a bit over the top for the home gamer...perhaps even a little intimidating....if one had never seen the FGC-9 barrel being made.

Longer (rifle length) mandrels as used with the fgc are not really practical, but what about using a 3d printed "button" similar to what's used in the commercial process...along with a couple stepper motors and an arduino? Any bore size or twist rate becomes instantly possible... Not an ecm setup, but the idea for the mechanical side of things is here...



Working on the mechanism as time permits. Rifling buttons will mount on a rod and copper foil "tape" fits into grooves in the button in place of the copper wire used with the fgc mandrels

ECM Head...mount for the stepper that turns the button and guides along a section of aluminum box extrusion with a slot cut into it...simple enough.



Parametric button model


FYI...4130 seamless tubing can be readily had in Canada from Aircraft Spruce. McKinnon Metals is another place worth checking out...for this and other materials for the budding machinist.

Regards,

iknowmy3tables wrote: ↑

Thu Sep 09, 2021 3:17 pm

there are plenty of carbide rifling buttons for sale on ebay around $50.

I've seen these but I was a little hesitant to purchase one, lest it's something flagged by the powers that be in the same way those wink wink nudge nudge "fuel filters" tend to be.

Found some stuff for Canadian residents. Small for low pressure pneumatics but possibly useful.

https://www.shawcustombarrels.com/shop/ ... blanks/270