Rail barrel, shooting BIG

B

Bob Kingsbury

New member
Not to far back, I shortened the barrel on my rail gun. It seems that
since It has been shortened, it rings like a tuning fork. Lifting the top
(moving half) off the base will always start it. Like a tuning fork, a
harmonic or similiar frequency could induce this into vibration. This gun
now shoots incredible verticle and no amount out tuning seems to help.
The barrel is epoxied into a barrel block and the block is bolted to the I-beam. My dial indicators say nothing is moving in any way. Just wondering
if my barrel length has become untuneable, Anyone seen this before
 
Bob

You could have hit some weird harmonic range. Contrary to what many think, having a big 1.450 barrel IS NOT a guarantee that the thing will shoot. I have a pile of Unlimited barrels that will not cut it in 10 shot Competition.

I have seen improvements in my Rail Gun by simply loosenning the barrel block and sliding the barreled action one way or another. That does change the vibration pattern. However, it is mounted in a Delren Sleeve as an isolator. Since yours is epoxied into the block, that is not an option, at least, not a convienient and quick one.

I would say to check the amount of torque on your barrel block, but having the block epoxied in makes that a moot point.

If it is harmonics, then you should be able to tune it out. But, if it is that finicky, it probably won't stay there.

Do make sure that your barrel is epoxied securly into that block. Any shift, one way or another, would play havoc.

Nothing is worse than a Rail Gun that won't shoot. I wish I couldgive you a definite answer........jackie
 
Jackie...

What torque setting do you use on your barrel block? Also, as far as the placement of the barreled action in the block, what has been your most consistent range of where you've found most of your good railgun barrels to work...as measured from the front of the action to the block?
 
If you are talking about "tuning" by making velocity changes then you should be able to tune the short barrel just as you did the longer barrel. However, you will now be working in a slightlydifferent velocity range with the shorter barrel so it may be necessary to work with powder charges you haven't had to use with the longer barrel.

If you are talking about tuning with a tuner then it may be difficult to add the weight necessary to affect the short stiff barrel. Most of the tuning effect comes from the weight addition rather than the position of the weight so you just might not be able to add enough weight to have much effect.

I think another reason for your problem may lie in what you have done at the muzzle. The higher pressure you now have at the muzzle would make the bore and crown condition more important than it was with a longer barrel. If you shortened from the muzzle then you might have changed the muzzle enough to cause a loss of accuracy. I'm not sure why this would add only "vertical" to your groups but it's something you might check.
 
Bob,

I'd say you have two options. 1. Cut the barrel shorter and hope to get out of that harmonic or 2. start adding weight to the muzzle. I'd say if you hang the same amount of weight you cut off out past the muzzle, it'll start shooting like it used to.

Kent and Bob,
I have almost no experience with rails but have been picking the minds of those who win with them regularly. Dwight (who won most of his HOF points with a rail) is really against shortening a barrel that shoots or shot good/great. He said he believes you run a high chance of getting out of a good harmonic range with the barrel (which has more to do than just tuning a load to get it back in) than you do with the barrel staying good/great shooting. That applies to rail/LV/HV or any barrel for that matter. He also said that rail blocks have been known to shoot good with between 35-80 inch pounds of torque. He said start around 40 inch pounds and go up 10 at a time to 80.

Also, like a lot of people have quoted Dwight in saying "Sometimes you just have to tune with an action wrench"

Hovis
 
Bob Kingsbury said:
Not to far back, I shortened the barrel on my rail gun. It seems that
since It has been shortened, it rings like a tuning fork...
Click to expand...

First, let me say I have NO experience with rail guns, but it could be that it rings like a tuning fork because it IS a tuning fork. By shortening the barrel, you have increased its stiffness and reduced its mass, which will increase its natural frequency = square root(stiffness/mass). If the gun's base has the same natural frequency, then you have exactly a tuning fork. If so, there are two routes to make it not a tuning fork: 1) Increase damping. Tuning forks ring so well because they have very little damping between the two forks. With a metal on metal barrel clamp (aside from the thin, I assume, layer of bedding), there is little damping. Adding rubber, plastic, or better yet, sorbothane between the barrel and the base might work. 2) Change the natural frequency of the barrel or base. Ironically, cutting the barrel off more, raising its natural frequency, could help. Reducing the stiffness or increasing the weight of the base, which would decrease its natural frequency, might work too. Or going in the opposite direction (increasing stiffness or reducing weight of the base) could do the trick. Easiest might be to lay a couple of bags of shot on the base and try it.

Let us know what you try and how it works.

Cheers,
Keith
 
Bob, you might try placing your hand on the barrel after each shot and see if that helps anything. That may seem like a strange suggestion, but I think rail guns quite frequently when shot fast have the barrel still vibrating when the next bullet is going down the barrel. Placing your hand on the barrel stops the vibration and gets you ready for the next shot. That won't help you with how it shoots, but it might help with determining if the barrel is acting like a tuning fork. Another thing you might do is wrap lead tape like golfers use around the barrel for quite aways down the length of the barrel. This will change how the barrel vibrates as well, but will do it while the bullet is going down the barrel instead of just after. It seems a lot easier to get a varmint class rifle to shoot than a rail gun. Rail guns seem to have a mind of their own sometimes.

When I had my rail gun mounted in a delrin sleeve, it seemed that the tighter I tightened the 1/4"-28 screws holding the barrel block together the better the rifle shot. My new barrel is mounted in a 2" aluminum sleeve and cerrosafed into the sleeve and then clamped into the barrel block. Jackie has gone to the delrin sleeve, I've gone to something similar to what Jackie had before he installed the tuner on his barrel. If you don't try it, you don't know which you like better. The delrin sleeve is certainly easier.
 
Tapered barrels for unlimited

I have never heard this kind of tone from a tapered barrel. If a cylindrical
barrel of the type used in Rails is prone to this, Maybe a 1.450 could be tapered forward of the blocks and eliminate some of this. My thoughts are
that if I can hear this , it is of low frequency and large amplitude. Which
would allow it to ocilate for a longer period of time, What say you ?
 
I have added a weight, clamp ring around the muzzle. This has changed the
sound quite a bit. It no longer rings the same. I may have choked the muzzle by doing this, another day at the range
 
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Mike's advice is useful

I had a barrel that I could shoot as fast as I could with no penalty. Next barrel wouldn't shoot well until I learned to pause a second after hitting the stop.

Cut a gasket from target paper to go between the block and the I-Beam. Do a good job such that the bolts don't catch on the paper and no "chads" are present to create a high spot.
 
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