Barrel porting vs barrel length

[shorty]

I don't think I have ever read anything about optimized porting vs barrel length. I have been wanting to ask about this but I feel a little dumb as I should know the answer.

We typically tell people to run 70%/80% porting for a given caliber but, is that porting size really optimal in regards to efficiency and power for a set barrel length?

I guess what has me thinking is my 25&22 cal synrod,PCPhacks Brod build, Motorheads 22 Mrod (max 64fpe), and some of those Prod builds.

When I did my 25 synrod first I had .196" porting (stock barrel). I could get into the low 90's for a shot or 2. I then went to .22 porting and I still can only get close to the mid to high 90's.
The positive thing with the larger porting was I can run a higher FPE at lower pressures but, the max was still hovering the 90's.
Almost the same thing in 22. With .168" porting I can do just over 60fpe max with 28 grain and up (stock barrel).

So,
Here comes PCPhack with his 25 cal bottle build (stock barrel length) with porting at "I think" .2" and he is now at just at 100fpe.

Then,
Motorhead comes in with some numbers from a 24" barrel in 22 cal with only 65fpe with full bore porting ( I would have thought it should have done 70+)

Guessing now,
With Prod barrels being much shorter, it would seem as if 75%/80% porting would not be optimal for efficiency and power because the barrel length is not there.

Geez,
I really know this is a dumb statement/question but is there a porting vs barrel length "rule of thumb" for efficiency and max power ? Like if you run a 16" barrel max porting should be 60% of caliber and 20" barrel should be 70% or 24" barrel should be 85% porting?

[nervoustrigger]

Well, I would say the ideal scenario is full bore porting and a super short dwell, regardless of barrel length.  But from a practical perspective, more than 75% of the caliber (56% when expressed as area) usually works well for pellets.  Any larger and an oblong barrel port is needed to prevent damage to the pellet.  And for “full length” barrels, this typically lets you get up to the useful upper velocity limit of 900 - 950fps with medium to heavy-ish pellets.  If the porting is too generous, it can actually become difficult to get the dwell short enough to keep the pellets from going too fast, or end up with a poor ES in the process.

For bullets or super heavy pellets, full bore porting always seems preferable.

[Motorhead]

Then,
Motorhead comes in with some numbers from a 24" barrel in 22 cal with only 65fpe with full bore porting ( I would have thought it should have done 70+)

Mine WAS NOT a full power test ... but one at the max stroke my SSG & current spring would allow on an intermediate weight hammer ... STATING in thread theirs more in there !
I have no doubt what so ever 70+ was very doable.



Now as too your inquire ... always felt those PCPs with big ports on high pressure when also set up for short dwell make maximum projectile acceleration & harness the potential of barrels best.
In such set ups added length really does not add much if valves already closed & projectile  is inside 1/4 to 1/3 of a typical length barrel. ( 16-20" range )

JMO tho ...

[Nvreloader]

Guys

To go along with Shorty's post,

I am going to throw out a question for all of you, as I for one don't really know this info, and would like to learn more,
and I am sure that this info would help those that are following this trail, would you give your personal spec's,
for these bbl TP holes sizes, using PELLETS and what do YOU consider High pressure, 1500, 2000, 3000 psi?

In the following calibers, what would the optimum size of the bbl port be, using the following info?

17 cal,
Round port size = .xxx" diameter and Oblong port size, width by length = .xxx" by .xxx",
22 cal,
Round port size = .xxx" diameter and Oblong port size, width by length = .xxx" by .xxx",
25 cal,
Round port size = .xxx" diameter and Oblong port size, width by length = .xxx" by .xxx".

Can you please use the decimal sizes, ie, .160" etc, and we also realize that the proper deburring is required.

Tia,
Don

[skorec]

At the first moment I had understood that this thread   is discussing about barrel porting/putting/inserting  into the gun breech because for higher barrel length barrel  is really necessary  to use stiffer fixing it  into the  gun .

If we are speaking  about the air transfer port hole size for particular barrel caliber - the ideal is 100%  but best is to do it  max. 70%.  If we  need  to increase FPE we  have to do oval-shaped hole only .
In another case the accuracy may be affected with   some kind of pellets.

[rsterne]

It all depends on what you mean by "optimum".... For maximum FPE, you want full bore-area porting, maximum pressure, and the longest possible barrel.... fed by a valve that stays open until the pellet or bullet leaves the muzzle.... Anything you do to reduce any of those variables will reduce the potential FPE.... but may also reduce the amount of air released by the valve.... Obviously, the best way to conserve air is to shorten the dwell.... Reducing the port diameter will reduce the amount of air released, but it does so by essentially reducing the pressure available to drive the pellet.... Generally, a smaller port will increase the FPE/CI, but only up to a point.... I think you will find that if you are in search of efficiency, you still want large ports, but reduce the dwell to reduce the power until you find your optimum....

Having said that, I don't think there is much point when shooting pellets of taking the trouble to make an oblong barrel port.... a drilled port of 75-80% of the caliber seems to work just fine.... Bullets are a different matter, mostly because of their mass, you need to move more air to drive them.... so bore-size porting will allow you to achieve a higher FPE.... I have not seen, nor can I think of any logical reason for there to be.... any relationship that indicates that a shorter barrel should have a smaller (or larger) transfer port.... IMO they are separate issues, with no correlation....

Bob

[JimmyD.]

One rule of thumb is take your pellet weight & speed. Add or subtract 10fps for every grain up or down, to 24”.

Once you pass 24” a PCP is able to gain 20+ FPS per inch. Which is why I wish guns like EDgun Supermagnum, Rainstorm,Sumatra or any other like new Raptor came in custom barrel size.

The Supermagnum had a 23.75” LW and reg at 112 bar had no problem sending 31.9 RBT slugs 940 FPS. Just imagine if it had a super long barrel. 28-36”

How did Lewis & Clarke’s gun get so many power shots on 800psi? .32 caliber bullets/round balls? Those men were used to Brown Bess, PA & Kentucky long rifles 4’ barrels. As a BR shooter who doesn’t mind being tethered I’m always looking for longer drop in barrels & want a Grizzly Gunsmith lathe.

[Ribbonstone]

Generally look at barrel length change as a proportion rather than an absolute per inch.  If you don't phart around with anything else while swapping barrels (and the urge to tweek this or that, or make some major change is strong once you have the PCP apart)...the increase in barrel length is more like a proportion.

Adding 4" to a 12"P-rod barrel (to 16") is likely to show more gain per inch than adding 4" to a 20" barrel (to 16") or 4" onto a 24" (to 28")barrel . Still 4" ion each case,but in the first case that's a 33.3% increase in launch platform...in the second it's like 20%...and the 3rd more like 17%.

NOT that you get that percentage of incr3ased speed...more like something between 1/2 to 1/4th the percentage of change.


So basically you gain MORE speed per inch if you start shot...less speed per inch if you start at a normal-to long length.

(And it works backwards just as well...cutting 4" off a 28" barrel is going to slow down things a lot less than cutting 4"off an already short barrel).