So, what rifle you be taking about there?
Thanks for looking. Asked to have a look at this, brand new, today. Ideally to reduce cocking effort and smooth out a bit. Not ideal start 'cos .177 and carbine barrel 10" maybe.
Bore 29 mm x stroke 77.5 mm TP a slack 2 mm drill fit and say 15 mm long
Spring 235 cm x 21.7 x 3.4 x 27 coil x 52 lb/inch Pre load 40 mm Sorry forgot the weight/guide flanges.
11 fpe, 8.5 gn Exact and 7 gn Hobby 10+ with Bisley 10.2 gn
Nice synthetic sleeve and steel guide 230g piston and 90g weight!!!
Last edited by deejayuu; 09-01-2021 at 05:26 PM.
So, what rifle you be taking about there?
S&W M&P 1522 - Anschutz 1416 - Anschutz 1761 - Beretta Silver Pidgeon III - Yildiz SPE-ME 12G - ATA SP Black 12G - FWB800 - FX DRS - HW110K - HW100 - Steyr Pro-X - Steyr LG1 - Walther LGM-2
Gamo 1250?
Quite a few variables here. You don't mention preload. At first, looking at the combined weight of the piston and its weight, one might be tempted to try it without the weight. But if we're running very little preload, the increased piston weight will help counteract bounce, bearing in mind the larger diameter piston's surface area. One might be tempted, if you have to play, to open the TP out to 3mm and use a spring with thinner wire but more preload? Losing that piston weight then might quicken the action up?
THE BOINGER BASH AT QUIGLEY HOLLOW. MAKING GREAT MEMORIES SINCE 15th JUNE, 2013.
NEXT EVENT :- August 3/4, 2024.........BOING!!
Pretty much on the ball, Tony.
It's a Lightning, I think. I'd replace the piston weight with a replica turned from acetal, fit a much softer spring with preload to compensate, turn an acetal guide to suit the spring, and look for circa 10 ft. lb. with the most efficient pellet.
I think I'd leave the transfer port as is. With such a short barrel, you want the mass air flow to choke earlier in the stroke.
Thanks everyone. It's a BSA Lightning current model so good shout on Gamo which may share the platform.
I don't know enough to fully understand but wondered if the basic dimensions are for +12 fpe output but strangled at TP for UK which then needs the "stovepipe" weight, which fits like richard in a shirtsleeve, and stiff spring.
Suggested mods. to TP and piston weight make sense to me and will be interesting if given go ahead.It's brand new after all.
P.S. some factory glitches present too including tight and rough edge seal as you see.
Was hoping he'd be along along with every one else of course. Updated TP length and PL.
For info. removed weight and output fell 2 fpe. Not surprising 'cos lost 6.5 mm at the flange. Still lower with Bisley !0.2 gn
Restored preload, only, with synthetic top hat as suggested. Very balanced across Hobby, Exact, Bisley but >1 fpe below original.
The transfer port length is quite handy.
What does the firing cycle feel like? Can you detect if it feels "quicker" than before? Any discernible difference in perceived recoil / surge?
Upping the preload to compensate for less piston momentum may help? Maybe start with +3mm and increase by 2mm increments?
THE BOINGER BASH AT QUIGLEY HOLLOW. MAKING GREAT MEMORIES SINCE 15th JUNE, 2013.
NEXT EVENT :- August 3/4, 2024.........BOING!!
Thanks again. Another snippet. Fitted a 270 mm x 36 lb/inch spring. No piston weight or washers so 60 mm preload.
Very balanced across pellets at 9 fpe. Same as original spring with no weight.
Not sure this is right but thinking original spring uncocked force 70lb cocked 70 + 160 = 232 lbs approx
test spring " 86lb " 86 + 112 = 198 lbs "
Which leads me to a trap of thinking similar energy out for less in so more efficient??? Marks out anything please
say the TP is 2.5x15
I'd defo open to 3mm or it just won't flow.
Normally Id say 3.2 (my PE is 3.2 x 15'and the accelerometer tests show it still being slightly too tight) but the 3.0 will cushion it a bit with the shorter barrel - although being .177 it will maintain higher pressure anyways,
and throw that weight away.
Removing the weight without opening the TP will kill the power, as its larger bore, and a tight port = way too much bounce.
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.
The fact that decreasing the spring energy with the test spring did not reduce the muzzle energy suggests that the port is choking the airflow too early, so the only way to increase the air flow mass and hence muzzle energy beyond the onset of choking is to increase the air's density (pressure) via the momentum provided by the piston weight.
As Jon suggests, enlarging the port seems the next step. This will delay the choking to later in the compression stroke, when cylinder air pressure, and hence mass air flow, is higher.
Many thanks again for the support. Hope you stay interested. Will try a bit less spring rate to confirm your thinking and report back.
Tempted to do an efficiency calc. just to see if I can get it right. All very interesting.