Tuning Tuning my Kral Bighorn for slugs

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Stretch

Stretch

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Jan 27, 2019
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Virginia, United States
I thought I'd start a thread to chronicle my efforts to tune my Kral Bighorn .25 cal rifle for slugs. If you're not familiar with the Bighorn, it's an unregulated, PCP with a bottle max pressure of 250 bar and a choked barrel. I purchased it several months ago from Troy Hammer who had already made some changes to the power plant (modified hammer and spring, modified through port). Here's my Bighorn as she is dressed currently. (No, she didn't come with a barrel band or carbon bottle, I added those.)

My Bighorn.jpg


So far I've polished the bore and slugged the barrel. When I slugged the barrel the choke started maybe an inch from the muzzle and felt very tight. When I measured the pellet, it was about 0.24". That seems very tight to me and I'm not sure what I'll do about it. I'm not keen to cut and recrown the barrel since I'll loose the threads at the end for the muzzle device. So until I decide what to do about the choke, I've decided I'll shoot FX Hybrids as I think they work best in a choked barrel.

I wanted to shoot over a chrony but ran out of 26 grain Hybrids, so I used 25.39 grain JSB pellets. And here are the results:

Bighorn chrony.jpg


On three occasions the shot velocity dropped 15 fps. I'm not sure what could have caused it. Defective pellets? Glitch in the chrony? Any other ideas?
 
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My NP02 when i first aquired it had an insanely tight choke. It was grouping one or two inches at 20 yards rested. I took a wool mop and put it on the end of my drill and smeared it with a mild abrasive paste and polished it on a slow spin. as the choke reduced i tested it several times and the groups were reducing. I didnt know when to stop really but I guess the choke is down by half and the gun is now one holing 5 pellets bull after bull after bull.
 
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Telephonepete said:
My NP02 when i first aquired it had an insanely tight choke. It was grouping one or two inches at 20 yards rested. I took a wool mop and put it on the end of my drill and smeared it with a mild abrasive paste and polished it on a slow spin. as the choke reduced i tested it several times and the groups were reducing. I didnt know when to stop really but I guess the choke is down by half and the gun is now one holing 5 pellets bull after bull after bull.
Click to expand...

I will have to think about that. I'm a bit concerned that using a mop at the end of the barrel may modify the crown... although I suppose if it does I can clean up the crown later.
 
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What does the main part of your barrel, without the choke, slug out to?

I found my Bighorn 25 barrel, minus that excessive choke, to be very consistent, however I still polished in a very long taper to the interior from Breach to muzzle using #7 ad finishing with Flintz. I am considering firelapping my new TJ barrel using Flintz.

Good luck to you Stretch.

Regards,

Roachcreek
 
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Roachcreek said:
What does the main part of your barrel, without the choke, slug out to?

I found my Bighorn 25 barrel, minus that excessive choke, to be very consistent, however I still polished in a very long taper to the interior from Breach to muzzle using #7 ad finishing with Flintz. I am considering firelapping my new TJ barrel using Flintz.

Good luck to you Stretch.

Regards,

Roachcreek
Click to expand...

That's a good question, although as I recall it felt pretty consistent to the choke. I'll have to do it again and see what the answer is.

I thought about firelapping, but having never done it before I wondered if there's a risk of some polishing compound getting into the transfer port?

Your input is always appreciated Roachcreek!
 
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Stretch said:
I shot 4 magazines at a 50 yds target this afternoon. Wind was 5-15 mph from my 5 o'clock. This was the best group. Shooting the JSB 25.39 at 950 +/- fps. Not horrible but also not great.

One thing I will say is that the trigger on this Bighorn is just where I like it. Very light and crisp.

View attachment 357749
Click to expand...
Seeing as how a 15mph wind will blow a 25gr pellet 4” off its mark at 50 yards, I think you and your gun are shooting great. With 10mph swings in wind velocity, that’s not easy to compensate for and keep things tight. Possibly competition level shooting right there.
 
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Vetmx said:
Seeing as how a 15mph wind will blow a 25gr pellet 4” off its mark at 50 yards, I think you and your gun are shooting great. With 10mph swings in wind velocity, that’s not easy to compensate for and keep things tight. Possibly competition level shooting right there.
Click to expand...

I shot 4 magazines (20 rounds) for 8 five round groups. By the time I got to the 8th target, I tried to shoot in the lulls as best as I could. I've found that if I rapid fire during a lull my groups tend to be smaller than if I just shoot one shot per lull. I'm going to have one more go at polishing the bore and cleaning up the crown to see if that helps at all. I'm still shooting pellets and won't have any slugs to shoot until later next week. That will be the real test.
 
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I spent time this morning polishing just the choke and then followed by cleaning up the crown (brass screw + Flitz). I also noticed that the hammer spring adjuster had moved on its own, so following Troy's advice I added a set screw to hold it.

Then I shot 5 magazines of JSB 25.39 gr to 50 yds, light winds. To my eye, the groups looked about the same as yesterday. I believe that I'm at least partly to blame. This rifle is very hold sensitive. I normally hold a rifle loose and let it recoil freely. But I noticed that my groups shrink if I hold the rifle firmly and pull it into my shoulder.

Here's my best group today. Again, not bad but also not great.

IMG_20230520_125817025.jpg
 
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@Roachcreek - I slugged the bore before and after the choke before and after this morning:

Before: main bore - 0.250" choke - 0.240"
After polishing: main bore - 0.250" choke - 0.243" +/-

Before today's polish, the head of the pellet was deeply scored after passing the choke. After the polish it's still scored but not as deep. To my bare (68 yr old) eye, the crown now looks very even and clean.
 
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