Velocity decrease with increase in vent size?

[mazo kid]

Here's a question I haven't seen discussed here: folks have said that velocity will decrease with a large vent size, has anyone documented this over a chrony? By this I mean noting velocity for a few shots at smallest vent size and then successively increasing the size of the vent while noting velocity change, powder charge staying the same of course. If this has been posted before, I apologize, but haven't been able to find anything by doing a search.

 

[paulvallandigham]

When the TH diameter gets above 5/64", you see a measurable drop in velocity, but its not so much that its worth worrying about. I don't know anyone who has bothered to investigate it, either. When I set up my flintlock- long before I owned a chronograph, I simply watched for a drop in the POI at 50 yds when I was opening the TH in my Chamber's White Lightning vent liner.

I do think the White Lightning vent line, having a parabolic CONE on the inside of the liner, uses escaping gases to keep other gases in check for a few microseconds, until the ball is moving. It does this by focusing gases and pushing them back towards the bore, during firing, slowing the escape of gases, uniformly.

Most liners come with a 1/16" or smaller hole in them. You can use either 3Fg or 2Fg powder in the barrel and get good consistency in your MVs. When you get over 1/16",some 3Fg powder will come out the TH, while most 2fg powder granules won't.

If you are only plinking, or hunting with your flintlock, the choice of TH liners simply won't be noticed. It is noticed by the serious Target shooters, however. Its the target shooters who have told everyone all the glowing reports about the Chamber's designed, White Lightning liners.

 

[longcruise]

I think Larry Pletcher may have tested this. Maybe he will add some insight.

 

[roundball]

Certainly seems like velocity SHOULD be less with a larger vent hole bleeding off more pressure.
Someone will verify before I'm at the range this summer with my chronograph again, but I'll still try to remember to sacrifice a vent liner and drill it out larger than normal for testing that

 

[Dphar]

It will also reduce accuracy.
Vent flow rates increase geometrically as the size increases and the increased gas escape will effect the velocity. Further loading a gun with a large vent blows out a varying amount of powder as the projectile is pushed down this is a further cause of velocity loss and increased variation.


Dan

 

[paulvallandigham]

Danis right. If you are using FFFg powder in your gun, use a TH liner with a hole small enough to keep powder from pouring or being expelled out the TH when your ball is loaded. Close the frizzen before seating a ball. Then tip the gun up to a double arm carry position, and slowly open the frizzen. If there is powder in the pan, the TH is too big for the granule size of powder you are loading.

Generally, its my personal experience that any Th that is larger than 1/16" will allow Today's Goex FFFg powder granules to be blown out the TH when the PRB is seated onto the powder. That small, unpredictable, and non-uniform reduction in the powder charge in the barrel does cause a wider SDV and larger group sizes, noticeable at 50 yds.

 

[Larry Pletcher]

I think Larry Pletcher may have tested this. Maybe he will add some insight.

marmotslayer,
I have done some testing with varying sizes of vent holes, but the variable was time and not velocity. The reason was one of convenience. We could time various vents in my garage, but to do chronograph test inside is a bit more tricky. If properly motivated, we could run those tests outside this summer.

Dan Ph. was a help the last time. His idea of a spanner wrench to make a removable WL lightning liner was a big help. In chronograph tests we'll simply redrill the hole. The spanner wrench comes in handy because the liner can be removed when testing is over. Or, to allow different liner interior shapes. It can be quite involved if you want to do all types of liner shapes.

I'll make no promises, but I can envision a MB article coming out of this kind of test.

Regards,
Pletch

 

[JayDee Flohr]

Further loading a gun with a large vent blows out a varying amount of powder as the projectile is pushed down this is a further cause of velocity loss and increased variation.

That would happen if you don't plug the vent. I always put my pick in the vent hole, then load. No powder loss. Personally, I like a large vent hole. It gives me good ignition and the guy next to me also. :thumbsup: :haha:

 

[longcruise]

Pletch, Hope you are able to do the test. Would make a very intersting read!

 

[ebiggs1]

Dan Phariss and paulvallandigham,
What are your thoughts on larger touchholes shortening flint life? I suspect they do. This is another of the little tests I am doing and I wonder if your observations are similar to mine?

 

[paulvallandigham]

I don't know how enlarged THs would shorten flint life. I don't own any gun with an enlarged TH. The largest TH I have is 5/64" on my .50 cal. rifle. the TH in the vent on my fowler is a bit smaller in diameter.

Are you suggesting that the blast of gases out of the TH is going to Melt the flint? I don't think so- not when you compare the melting temperature of flint( silica) to the brief flow of heat and air coming out of the TH.

If the cock goes so far forward that your flint's edge is down in the pan of the lock, when the hammer stops, you can weld, or even glue leather on the face of the hammer stop to shorten the movement of the cock, and keep the edge of the flint away from the TH. A cock need not put the edge of the flint down that far to ignite the powder in the flash pan. In fact, if the angle of Impact of the flint to the face of the frizzen is located properly, and at the correct angle, sparks should be created and THROWN down into the pan ahead of the edge of the flint, so that ignition of the pan, and even the main charge begins before the cock hits its stop.

 

[hanshi]

I've wondered about this problem for a while, now. On every flint gun I own the flint leading edge next to the vent is almost always dulled and polished while the rest of the edge is still fine. This, of course, is after a couple dozen rounds or so. The only thing that comes to mind is that the high velocity particles escaping through the vent hole "sandblast" that area. It's not really a problem, per se, just requires a bit more knapping. All the liners I own are drilled out to 1/16". I don't like anything larger but smaller is okay as long as it works.

 

[ebiggs1]

OK bear with me on this one. The largest touchhole liner I have is also 5/64th.
I have several with a number 5-drill hole. And just a couple with 1/16th. My favorite size is the number 5-drill hole, I might add. I am trying to test weather touch hole size is affecting the flint. One thing I have noticed is when you dry fire the rifle the flint seems to stay sharp longer that if you are shooting the rifle. I have tried this on about ten different guns now and it seems to be true. I am not far enough along to say for sure but I am still looking at it. Thoughts?

 

[JayDee Flohr]

Thoughts?

Your tests will be inconclusive. Every flint is different and with that being the case, a valid test just can't happen.

 

[roundball]

Further loading a gun with a large vent blows out a varying amount of powder as the projectile is pushed down this is a further cause of velocity loss and increased variation.

I like a large vent for the speed...mine are an eyelash under 5/64"... .072" diameter as I recall...and are very fast.

I don't plug the vent when seating a ball and in chronograph tests with Goex 3F, SD has been as low as 6 fps so I'm apparently not losing any powder to speak of, and I don't believe any velocity difference of consequence would come from that...it would come from pressure loss out a larger vent hole...same principle as when a nipple hole burns out larger

 

[KHickam]

Why does it matter? If your gun with a larger than normal touchhole (whatever that might be) shoots accurately and the load generates enough power to kill cleanly - who cares what the velocity is?

Sometimes I believe people overthink things :idunno: :hmm:

 

[ebiggs1]

Maybe, but if you watch long enough, you do tend to notice a trend. Right now with a multitude of trigger pulls, it seems, as if dry firing does not damage the flint as quickly as actual firing the gun does. I have tried to be as random as possible knowing the properties of muzzle loading.

 

[JayDee Flohr]

dry firing does not damage the flint

That speaks volumes. I put a flint in the gun, try it once to check spark and it never gets dry fired after that. I have better things to do than piddle fart around wondering about trivial things like this, as it has absolutely no bearing on the grand scheme of things. I have complete faith in all my flintlocks to perform flawlessly.

 

[Tom Compton]

On some guns the flint may not get in the gas blast from the touch hole. On others gas cutting will certainly erode the flint on the edge nearest the barrel.

I have seen vent liners gas cut enough to lower the point of impact. Nipples on the other hand cause other problems before POI shift such as half cocking the gun and debris in the shooter's face.

 

[LRB]

Dan is correct. As a former driver/engineer firefighter, we were taught that flow increases 4 to 1 in hose size. So for every bit that you open a vent hole, you will get four times that amount of gasses coming out. Just my opinion, but if a 1/16" vent hole is not reliable, there is another problem. All the 1/16" vent holes with a reliability problem I have encountered, were corrected with an outer cone into the vent hole, including a new vent I recently installed. I had modified the inner cone to the max size I trust, but was getting many pan flashes with no ignition. In haste, I had forgotten to cone the outside. After doing so, I have very reliable ignition. As far as the inner edge of the flint, I have never noticed it being worn away faster than the rest, but as long as you get good spark, who cares.