.25 Disco fever.... add another project to the pile

[lloyd-ss]

Bob, 1.08 lbs for the tube (1" x .065 x 20", 4130-normalized- surprisingly easy to work with.) + about .3 lbs for the plugs.  Total weight is about 7 lbs, but I don't have a good scale in that range.

Total spread is 8.2% of the average, and if you drop the last 2 shots it's 6.8%. I would think that fixing the air leak at the transfer port would improve that.  What do you think?
I am surprised at how flat the string is, despite the bumpiness.  I guess the chamber volume and spring and pellet, etc., must all be matched nicely.  Luck.  I am going to go for higher power, though.

Next time I think I have an easier way to join the tubes at the front..... maybe.  I moved the fill nipple to the lower tube to make room for a shroud.  But if I use a tall breech block for a 25 cal marauder mag, it might be a moot point.   Thoughts?  I'll email the spreadsheet.

Lloyd

[rsterne]

It's possible that the air leak may be the cause of the slightly jagged shot string.... They are seldom perfect.... When I'm working on shot strings I use 4% for my cutoff point.... That is about 1/4" POI change from the velocity at 50 yards.... some people use 3%, and some (for 25 yards and under) use up to 10%.... I adjust my fill and refill pressures (and hammer hit) to achieve that 4% curve....

I've found that lightening the hammer helps even out the shot-to-shot variation.... The downside is that you lose velocity and energy and have to lean on the hammer spring to get it back.... Restricting the transfer port knocks down the maximum velocity which flattens the curve.... Opening up the ports tends to make the curve steeper, hence shortening the portion within the 4% window....

7 lbs. for an 1800 FPE total gun is pretty darn good, I think.... Do you have a small feed hole between the bottom and top tube so that the shot is only drawing from the upper tube?.... What are your port sizes right now?....

Bob

[john]

Using PA's specified 31grain per Baracuda.25, instead of 30.1, get even better ave fpe...~40.6fpe? 

I see that 2200psi -1040psi = 1160psi drop = 80Bar drop from a 310cc = 18.92cu in reservoir
but i don't understand where the 1512cu in came from...?...

[rsterne]

I assume Lloyd weighed the pellets, the manufacturers weights are often wrong.... If you multiply 18.9 CI by 80 Bar, you get 1512 CI of air at 1 Bar pressure, which we use for a standard to compute the efficiency....

Bob

[lloyd-ss]

Bob, Thanks for the thoughts on the spread.  The air leak will be the first thing I attack.  The deburring in the spring and hammer area.  We'll see how that helps and go on from there.
There is a .087 hole between the tubes, and that flows from the lower tube into the part that used to be the gauge adapter piece that has the 2 o-rings.  There is also a .087 hole thru the end of the gauge adapter to the front of the original tube.

[rsterne]

Gotcha.... and what diameter are the valve, transfer, and barrel ports?....

Bob

[lloyd-ss]

Using PA's specified 31grain per Baracuda.25, instead of 30.1, get even better ave fpe...~40.6fpe?  

I see that 2200psi -1040psi = 1160psi drop = 80Bar drop from a 310cc = 18.92cu in reservoir
but i don't understand where the 1512cu in came from...?...

John,
Yes, I usually weigh a few pellets before testing.  The heavier the pellet the farther off they are sometimes.  I just weighed 6 more of the H&N Barracudas  together and got an average of 30.3gn for those.  The tin has 31.02 printed on it.   

I think Bob explained where the cubic inches came from.... standardizing the volume of air used back to sea level pressure, kinda like air compressors are rated in standard cubic feet per minute (scfm).

There is a link to a shot efficiency calculator near the top of the page in the GTA Library that you might find interesting.
Lloyd

[lloyd-ss]

Gotcha.... and what diameter are the valve, transfer, and barrel ports?....

Bob

Bob,  I'll verify the dimensions when I tear it down for pics.

[Rocker1]

  You guys just kill me I look forward to your post but I understand about half of what you talk about but I am learning so keep it up.  You 2  are in a league of your own  no doubts about it . Thanks David

[lloyd-ss]

You guys just kill me I look forward to your post but I understand about half of what you talk about but I am learning so keep it up.  You 2  are in a league of your own  no doubts about it . Thanks David

David, you aren't by any chance calling us airgun nerds, are you? LOL    Bob, what do you think? 
Lloyd

[airpuffhunter]

Lloyd  were did you get the .25", 30" long barrel?

[rsterne]

~lifting up the flap of my shirt pocket protector to expose the very worn badge underneath~

YUP, it says NERD alright!.... 

Bob

[lloyd-ss]

Lloyd  were did you get the .25", 30" long barrel?

It is a barrel liner I bought from Mike Sayers, TJ's Enterprises, in Alexandria KY.  He does the actual hammer forging of the chrome moly barrel liners that various places sell, including Track of the Wolf, I believe. Bunches of different calibers and rifling configurations to choose from.  Sells it cut to length by the inch.  Easy guy to work with and I probably have 8 different barrels from him.
I posted a picture of a stock list of what he had  a while ago.  Maybe search under my name and Mike Sayers.

Bob, do you by any chance have a new price list that you could post?  I am pretty sure mine is out of date.

Lloyd

[rsterne]

I don't have a new list, but I can tell you that the current price is $4.25 per inch.... Here are some of interest:

.25 Auto: .243 bore, .250 groove, 1 in 14 twist, 6 land, 7/16" OD (will fit Crosman breech to convert to .25 cal)
.30 cal: .300 bore, .308 groove, 1 in 20 twist, 4 land, 1/2" OD (he also has a 10" twist - 6 land)
.357 cal: .348 bore, .357 groove, 1 in 16 twist, 6 land, 1/2" OD
.40 cal: .400" bore, .408 groove, several twists 14-24", 6 land, 5/8" OD
.45 ACP: .444 bore, .452 groove, 1 in 16 twist, 6 land, 5/8" OD
.45 Colt: .440 bore, .452 groove, 1 in 16 twist, 6 land, 5/8" OD
.45-70: .450 bore, .457 to .458 groove, several twists 17-26", 3 to 6 lands, 5/8" OD
.50 cal: .500" bore, .510" groove, 1 in 20 twist, 6 land, 11/16" OD

I just purchased a .457 with a 1 in 26" twist.... It has very narrow lands and looks like a great airgun barrel.... You can contact Mike Sayers at:

T.J.'s
3652 Neltner Road,
Alexandria, KY, 41001
(859) 635-5560
mikeandsusansayers "at" fuse "dot" net

He isn't set up for plastic or PayPal, you will need to send him a money order.... Liners are well packed and shipped promptly.... and he will do custom ones if you want to pay for a Mandrel or purchase in quantity....

Bob

[airpuffhunter]

thank you both

[john]

I managed to shoot a 46 shot string with the .25 cal Disco with 310 cc of reservoir and 30" barrel.
30.1 Barracudas, 46 shots, starting at 2200 psi and shooting down to 1040 psi.
Average FPS = 768, 8.2% vel spread, Std Dev 15 FPS,   Average FPE = 38.4 Std Dev 1.5 FPE.
The string is a little ragged because of the air leak, but...Lloyd

.... Excellent efficiency as well, 39.4 x 46 = 1812 FPE from 310/16.4=18.9 CI with an 80Bar drop = 1512 CI for 1812fpe/1512cu in = 1.20 FPE/CI....What does the gun weigh?....Bob

I see that 2200psi -1040psi = 1160psi drop = 80Bar drop from a 310cc = 18.92cu in reservoir...but I don't understand where the 1512cu in came from...?...

If you multiply 18.9 CI by 80 Bar, you get 1512 CI of air at 1 Bar pressure, which we use for a standard to compute the efficiency....Bob

As I read and reread that last sentence, I kept seeing "volume times pressure equals pressure" which of course is nonsense...what the...what is he...?...  

But that's not what you're saying.
You're stating a proportionality...a:b = c:d...or...a is to b as c is to d...
You're saying "X IS TO 80Bar AS 18.9 IS TO 1Bar so solving for X...X = 18.9cu in * 80Bar = 1512cu in.
In math, X:80 = 18.9:1     or     X/80 = 18.9/1   or   X = 80*18.9/1 = 1512

But why is that proportionality meaningful? Because it answers the question "How much air IS TO the reservoir pressure drop AS the original unpressurized tank of air WAS TO atmospheric air pressure (which is 1Bar at sea level)?" or "How much air is pumped into this tank to raise the pressure in the tank from atmospheric pressure up to 80Bar?"

So you're saying "Before we pump up the tank, there is 18.9cu in of air in the tank at normal atmospheric pressure...because 18.9cu in is the physical volume of the tank. If we pump up the tank to 80Bar, there will be enough air in there to fill a 1512cu in volume when allowed to escape back into the atmosphere...which is exactly what Lloyd did when he allowed it to excape one shot at a time 46 times.

Of course Lloyd didn't start down from 80Bar. He started at 2200psi = 151.7Bar and dropped 80Bar down to 152-80=72Bar = 1040psi...same deal...shooting 46pellets released 1512cubic inches of air.

If only that were as clear as it is wordy, Everyone would understand what I think I said.  

p.s. here is a pretty user-friendly Units Conversion website:
www.convert-me.com/en/

[lloyd-ss]

John,
You've got the hardest part figured out, and that is "How much "room air" did I use for those 46 shots?"
So those 46 shots used 1512 cuin, which is less than one cubic foot.  Total FPE for the 46 shots was 1814fpe, so that means that I got approx 1.2 fpe for each cubic inch of air used. Pretty amazing how these pcp's work, when you think about it.  That 1.2 fpe per cubic inch is one measure of an airgun's efficiency.
Glad you are enjoying this!
Lloyd

[rsterne]

shooting 46 pellets released 1512 cubic inches of air.

YUP, that's exactly what we are saying.... and that 1512 CI of air, as it expanded in the barrel, produced 1812 FPE of energy.... Hence the efficiency was 1.20 FPE/CI.... Here is a calculator you can play with....

http://calc.sikes.us/1/

HTHs....

Bob

[john]

Bob,Lloyd,etal:john,etal  =  LXA:glahsshoppahs

Thanks for the priceless lessons, Guys, i'm hanging up for tonight, ...zzz...hammers springs ports...zzz...valves...regulation...zzz...

The League of Extraordinary Airgunners

[lloyd-ss]

Well, some good news and not so good news on this project.
First off, I got the bumpiness of the velocities in the shot string really smoothed out nicely!  It turned out to be mostly about some partial threads in the rear cap (velocity adjuster/spring length adjuster) that the spring was snagging on when the gun fired.  I bored that out to give full clearance and then put a polished center guide for the spring to keep it straight during compression. Now the spring will compress and expand very smoothly.  I also found some damage on the valve head so I made a new stem and head. Shot-to-shot consistency is now very good (I don't have a full string because I was trying different pellet weights but here are five 31.4 gns that I shot: 865,864,865,861,859).

The bad news for me right now is that I seem to be stuck at 53 FPE, and I had to use 42.7gn EunJins to get that.  I opened up the passage to the front of main air tank to get good flow (previously it was a drilled .087 hole), but I have to figure out what the bottle neck is.  I will read around on the GTA and see if someone else has gotten thru this barrier with a disco. I feel sure somebody has.

Here are pics of some of the parts and pieces.  The transfer port from the lower tank to the upper is a .087 hole thru the modified phillips head screw that ties the tanks together.  The actual hole under the valve head is .250.  The transfer port bushing into the barrel is .156. Looking at the pics, one thing I haven't checked recently is whether the bolt probe is long enough to push the pellet fully past the transfer port.
Lloyd