BSA for H&N slugs

[rkr]

The introduction of H&N slugs got me rather excited, finally a small caliber bullet that can be fed from magazine and doesn’t take ridiculous amount of power to get up to speed. Even better they seem to work on standard .22 airgun barrels and provide rather good accuracy. Sounds like I need some of those … wait … I have only Evanix AR6 pistol that’s in .22 caliber and it’s not exactly the best gun for long range accuracy. After some thinking I realized that I do have two good .22 BSA barrels on the shelf and the parts bin has about enough parts to put together yet another BSA gun, so I decided to build a BSA parts bin special to shoot those H&N slugs.

Most of the parts I had came from a Gamo Dynamax that I bought partially broken years ago, 425mm air cylinder, breech block and trigger unit. I also had misc. parts new and used and I ordered some more until I had enough to make a new gun. The barrels I have are from 2004 BSA Hornet and 2009 BSA Sportsman(?), both 15” long so the power will be limited by barrel length. The 2004 barrel is excellent, very smooth and nice choke but not threaded for LDC. 2009 barrel is rough but has threads on the muzzle. Both barrels give excellent groups with pellets, 1.5mm ctc at 20 yards. Since I’ll be shooting bullets I decided to use the old barrel as it is smoother. Both barrels slug at 5.53mm at muzzle so I need to test whether .217 or .218 is the preferred slug size. I also have a .217 NOE sizing die in case I want to experiment with in between sizes.

Starting with the breech, Gamo Dynamax breech is for MMC action so I made a slot underneath to allow conversion to a bolt action. I also had an aluminium probe bearing I had bought and I made some mods to the .22 probe. A Teflon sleeve to give it a bit more support and to take slack out and some material was removed from the banjo to allow more turn angle for the bolt for improved locking (bolt popping open is typical problem for high power BSAs).
probe by abbababbaccc, on Flickr
banjo by abbababbaccc, on Flickr
 The bolt locking in BSAs is handled by a steel ball + spring in a drilled tunnel underside the breech. When you remove the breech from the tube these typically drop out and happily bounce around to hide in some hard to find corner. I replaced them with one of those spring loaded ball point set screws, all in one compact package and easy to adjust.
20180606_121545 by abbababbaccc, on Flickr
Another problematic thing is BSAs is the magazine locking bar, it is also held in place by a ball and spring, only they are really tiny ones and once they are freed up and bounce off the table they are extremely hard to find. In this case I made the locking bar “bolt action” style. It works but I may have to rethink it. At least there are no spring and balls flying around now when I lift off the breech.
20180606_121425 by abbababbaccc, on Flickr

In addition to those mods I also added a longer side plate that is held in place by four screws instead of two. The BSA multishot breech is really thin above the magazine and it is very typical for it to flex at that point. A longer sideplate will help to prevent unwanted flex that is often encountered in high power BSAs.
Since this is a BSA I just had to add a “Blast tamer” to kill the hammer bounce. I also wanted to have the MMC cocking system in place since I had one extra lying around and it’s a nice feature to be able to de-cock the hammer while leaving bullet in the breech. Things get a bit cramped with MMC linkage in place but I managed to fit the BT in. This design increased the shotcount in my Dynamax by 30% so it works rather well.
BT by abbababbaccc, on Flickr
On to the power department. I had previously experimented with those 15” barrels and managed to get 35 fpe so I knew that was possible with high enough pressure. One of the local lads had tried H&N slugs in his Lelya and was getting good accuracy at 36 fpe so I decided to set 35 fpe or a bit more as my power goal.
I wanted to have a regulator so I could use the 230 bar fills that’s the recommended fill pressure and I also wanted to have a gauge unit, so off to HUMA website and order of gauge/fill unit for BSA and regulator for Lonestar was made. I decided to start with a 140 bar pressure setting and go up if needed. The airpath was opened up to 4.2mm ports, up from the standard 4mm to get slight increase in power. That involved only drilling of TP, barrel port and the probe nose, no mods to the valve system at this point. Multishot BSAs are limited in power by the hollow probe nose design and that 4.2mm fits in there without any weakening of the .22 probe nose. Stock Scorpion hammer with PEEK tip and 1.3mm valve spring was installed as a starting point for power testing.

While power is needed it comes with loud bang and I want my guns quiet. For that I wanted a big shroud and while BSAs can take 28mm diameter shrouds I wanted something even bigger. It was time for breech risers. I took some 4mm thick plexiglass, made rectangles that fit under breech halves, dipped it in boiling water until softened a bit and clamped those rectangles in between breech and the hammer tube. After they cooled down I had two formed pieces that were marked by breech and tube openings, ready to be drilled out and filed. In addition to those risers I made a longer transfer port and longer hammer latch pin and I was ready to go. With those risers I can use a 35mm diameter shroud and that’s plenty for this power level and has actually twice the volume of a 28mm shroud.

So I went and put the action together and started testing with 21.3 grain barracudas.
action by abbababbaccc, on Flickr
1.3mm spring gave me 805 fps with hammer spring all in. That’s OK but not enough. I then tried different heavier springs and added weight to the hammer but couldn’t get past 815 fps ☹ At this point I have two options, either increase the regulator pressure or increase the air path size to 4.4mm (10% increase). Increasing pressure will lower the shot count so I decided to try the airpath first. Local HW stores did not have correct size drill bits so I ordered online and need to wait for a couple of days for them to arrive. To be continued.

[rkr]

I drilled ports to 4.4mm and that is IMO the largest size that a .22 BSA probe nose can be drilled to. Speed with Baracudas was up to 835 fps which unfortunately still falls short of my goals. So, as a next step I'll up the regulator pressure to 150 bar, enlarge the valve tunnel to 5.6mm and smooth the corner from tunnel to port as much as possible. The plenum in Huub regulator is only 13cc so I guess that's the root cause for this lack of power. It's too bad as others are getting 900 fps with 140 bar and this barrel length

[rkr]

Last steps of tuning. I ported the valve housing as much as I dared and to my surprise the valve tunnel was already at 5.6mm diameter so no changes there. Regulator at 150 bar was giving me 850 fps max. with Baracudas, that's 34 fpe and although close not what I wanted. Next one was upping the pressure to 160 bar (max. pressure on the regulator) which netted me 865 fps as I finally reached the 35 fpe mark. This is again a wall that I hit and I can reach it with normal BSA valve spring, swapping to a more heavy duty spring makes cocking really stiff but gives no more speed. It looks like that a bit over 35 fpe is all that this Huma reg can provide really, too bad as it was marketed as a regulator for .25 Lonestar and I was assuming 45+ fpe

Next thing is to put the gun together and see how it works with H&N slugs. I still need to build a shroud for the gun so that may take a while.

[rkr]

About the reg, I realized that Lonestar has actually 9" longer barrel than this gun so in a .25 Lonestar this regulator may actually reach 45 fpe. Too bad my .25 Scorpion has only 18.5" barrel making this regulator not a very good option.

[rkr]

A short update, I was later testing this same action with the same tune using 28" barrel and was hitting 48 fpe. That same barrel in unregulated gun with barrel size porting made 74 fpe. Show the effects of barrel length, pressure and plenum size. One more thing, short slugs jam the BSA magazine - 25 grains and up work.

[wiely]

Could we get an update? Really wanting to know which size slug works and how accurate they are.

[rkr]

Could we get an update? Really wanting to know which size slug works and how accurate they are.

It seems that BSA barrels do not like H&N Slugs. I tried both sizes and all weights in two BSAs, one with 10" barrel and the one in this thread. The best I could achieve was about 10.5mm ctc groups with .218 heavy slugs at 20 yards while pellets shoot 1.5mm ctc groups. I'm currently playing with a FX slug liner in a BSA, unfortunately winter has prevented any proper accuracy testing.