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diipii · Member

As a newcomer to airguns I am intrigued by the use of choked barrels.
I remember reading that the Germans had a cone-bored anti-tank gun in WW2 to increase muzzle velocity.(see sPzB 41 on Wikipedia)
Would some kind soul please enlighten me ?

fritz · Silver Status Member

A choked barrel just means the bit right before the muzzle is constricted. This will pinch any projectiles going through it, making them all the same size.

broommaster2000 · Moderator

Removing a choke can sometimes (not always, mostly when you're lucky) increase accuracy with one or two certain pellets, and it can also increase pellet speed since it's not slowed down by the choke.

I have seen this on a few Weihrauch rifles which were shortened, effectively making them a K version. Mr. Green

Slavia · Moderator

"K" for the German kurz, or short.

donec · Senior Member

Ok now you guys have confused me again. I thought the use of a choke was only for bunching multiple projectiles together to make a tighter pattern. Now it sounds like a choke can be used with a solid projectile to increase the velocity of the projectile, kind of like reducing the diameter of a water hose to increase the pressure. Is this correct or am I missing some thing? If it is correct can someone explain how it works?

Alstone · Moderator & Site Supporter

It can be a complicated subject Chokes, have a look here:

http://www.airgun.co.uk/Airgun_Accurracy.html

donec · Senior Member

Lipoly · Veteran Member

donec · Senior Member

Teryx · Veteran Member

I'll take a stab at it. Basically the choke in an air rifle barrel is an attempt to deal with inconsistency in the pellets we shoot. They are inconsistent for two reasons. First is that they are made cheaply by swaging, second is that they are soft and easily deformed. They very in diameter and in shape. Both of these things can effect the ballistic coeficient of the pellet. Changes in BC can cause vertical stringing since the high BC pellets will carry better and drop less. The variance in size can also effect the degree to which the rifling inscribes the pellet. This can produce varying degrees of stabilization which can cause random fliers in any direction. If you have a "choke" in the barrel's muzzle that is small enough to make each pellet a swaged fit through, then each one will leave the same size and with the same inscription from the rifling. In effect the choke is a pellet sizer. You don't see chokes in powder rifle barrels because the bullets are made to very high standards and they are jacketed with a harder copper skin that maintains the shape of the bullet. They don't need to be resized. You actually see some similar accuracy issues with cast lead rifle bullets. Those bullets are not expected to have the accuracy that pellets do though, so they just don't worry about it.

Alstone · Moderator & Site Supporter

Ok then a barrel that is choked at the front end squeezes the pellet by a small degree, this as the effect of stabilising the pellet in the bore, and getting better control while it’s travelling at it’s maximum speed, this may decrease the velocity by a small amount depending upon the length of the choke.
So all pellets fired by this gun will now have similar flight characteristics, apart from the weight which may vary from pellet to pellet.

Al

Teryx · Veteran Member

Alstone · Moderator & Site Supporter

[quote="TeryxI'll tell ya though, it makes me cringe to think of that pellet accelerated to top speed and hitting a choke. I have some barrels here that are choked .003 under the bore size and that is one tight squeeze. That has to dissipate alot of energy.
Teryx[/quote]

I think that is where it' gets complicated, if the choke is long enough the pellet will pick up the lost velocity especially on a pcp, also if the lead into the choke is gentle it would help on a Springer.

I think they use tight chokes on high velosity rifles, especially ones with a high twist rifling.

Al

Amigo · Veteran Member

Alstone:

Thank you very much for providing that link Exclamation

That has to be one of the most thorough, accurate and well written pieces of airgun info I've seen in a long time Exclamation

If there is a lack of understanding, I would respectfully suggest that a re-reading may be in order - not just a quick scan, but taking the time to do a real study.

Maybe a reader of that info will think twice before hacking off the choked end of a barrel. It also seems many of those who do shorten choked barrels successfully have & use compressive tools to re-choke those altered barrels.

On my first reading of the info, the one detail that I noticed missing was a reference to the "shuttle cock effect" as an aid to stabilizing the diabolo shaped pellet and thus reducing its tendency to tumble. Shuttle cock - think "badminton birdie".

As to the original question regarding the choke slowing the pellet.
There are a couple of reasons why I am not convinced that there is noticeable slowing.
1. Especially with smaller skirted pellets, the choke may allow improved pellet skirt to rifling seal such that the pushing air flow becomes more efficient due to reduced leakage past the pellet & thus possiblely increase pellet velocity.
2. Any momentary slowing as the pellet enters the choked region would allow the expanding air behind the pellet to rebuild pressure and accelerate it as it left the choke portion- just because the pellet has traveled part way down the barrel does not necessarily mean the piston has already reach its forward limit of travel.
3. If any remember and have used bean shooters - think back & recall some of the techniques used to gain both bean speed and accuracy.
Now SShhh... don't spilled the beans on those secrets.

BTW, the boxed Crosman Premiers are marked as to which of the multiple dies they were molded/cast from - Crosman keeps the separate Premier die runs segregated.

Al, thanks again, that was such a good reference I have done a copy & paste special to a MS Word doc. and saved using the HTML option

Amigo

donec · Senior Member

Hey guys you got the point across fine I feel like I kind of understand now how the choke is used on a solid projectile especially with air guns. Now I wonder if all air guns are choked or not and if mine is. I have a cheap set of calipers so I guess I'll try and measure both ends of the barrel and see what I get I just have to remember to measure both the same and not get into the rifling on one end and not the other.

Thanks a million and I agree the article was great just wasn't quite what I needed for just the choke concept, maybe too deep for me.