This article was written by Ed Doonan in the Mar/Apr 1999, #138 issue of the Fouling Shot.
In so much of cast bullet shooting we violate the first rule of solving engineering problems - we don’t understand the problem. So it is with gas checks; we know the effect gas checks have, but we don’t know the how or the why. Until we understand the HOW and WHY it is nearly impossible to make improvements except by blind experimentation. There are shooters that simply don’t care —- they may be the smart ones as it is very difficult to change the present system. Still at matches you hear shooters speculating on the effect of gas checks and plotting ways to make them better. A lot of this is generated by the jacketed bullet shooter’s experiments on wall thickness of jackets.
There are many theories on how gas checks work. Some of the more common sense ones are; 1. It strengthens the base of the bullet, 2. It provides a gas seal, 3. It is a scraper.
There are some things we know for sure. Plain base bullets are very accurate. If you are ever in a match with a good plain base bullet shooter and the wind doesn’t blow too hard he will teach you. We all know that at some point you must put on a gas check to increase velocity with accuracy. The only exception to this being the paper patched bullet. There are plenty of very, very smart plain based bullet shooters. If there were a way around this they would have found it by now.
Attempts to protect and provide a gas seal for the base of the bullet using a wad have been made from the start. There’s nothing new about a wad. Nearly everything that could possibly work, and some that couldn’t, has been tried. Wads appear to allow a slight increase in velocity with plain base bullets. Yet it is apparent that any kind of wad or filler does not do the same thing as a gas check. After shooting thou-sands of wads I finally ran a test where I could prove that the wad hit the bullet after it left the bore of the rifle. Never in those tests was a load with a wad more accurate —- they were either as accurate or less accurate. The slight increase in velocity simply wasn’t worth it.
Another theory is that GCs are a pressure seal and scraper. I think this is correct, never the less I think all bullets leak pressure before they can start to move. I’m not sure about soft lead breach seated bullets at low velocity and pressure but I think this is true for fixed ammo shot from a leade at 1600 f.p.s. and above. Here we have a bullet sitting perfectly still and suddenly it is hit in the butt by 30,000 to 60,000 Ibs. of pressure that wants to accelerate to 1600 / 2500 fps. The gas is not at the speed of light but it is pretty darn quick. No bullet sitting in a leade is a perfect fit. If it was it would be as hard to pound out of the leade as it is to pound on down the barrel. Since the bullet is not a perfect gas seal it simply has to leak some before it can start to move.
In so much of cast bullet shooting we violate the first rule of solving engineering problems - we don’t understand the problem. So it is with gas checks; we know the effect gas checks have, but we don’t know the how or the why. Until we understand the HOW and WHY it is nearly impossible to make improvements except by blind experimentation. There are shooters that simply don’t care —- they may be the smart ones as it is very difficult to change the present system. Still at matches you hear shooters speculating on the effect of gas checks and plotting ways to make them better. A lot of this is generated by the jacketed bullet shooter’s experiments on wall thickness of jackets.
There are many theories on how gas checks work. Some of the more common sense ones are; 1. It strengthens the base of the bullet, 2. It provides a gas seal, 3. It is a scraper.
There are some things we know for sure. Plain base bullets are very accurate. If you are ever in a match with a good plain base bullet shooter and the wind doesn’t blow too hard he will teach you. We all know that at some point you must put on a gas check to increase velocity with accuracy. The only exception to this being the paper patched bullet. There are plenty of very, very smart plain based bullet shooters. If there were a way around this they would have found it by now.
Attempts to protect and provide a gas seal for the base of the bullet using a wad have been made from the start. There’s nothing new about a wad. Nearly everything that could possibly work, and some that couldn’t, has been tried. Wads appear to allow a slight increase in velocity with plain base bullets. Yet it is apparent that any kind of wad or filler does not do the same thing as a gas check. After shooting thou-sands of wads I finally ran a test where I could prove that the wad hit the bullet after it left the bore of the rifle. Never in those tests was a load with a wad more accurate —- they were either as accurate or less accurate. The slight increase in velocity simply wasn’t worth it.
Another theory is that GCs are a pressure seal and scraper. I think this is correct, never the less I think all bullets leak pressure before they can start to move. I’m not sure about soft lead breach seated bullets at low velocity and pressure but I think this is true for fixed ammo shot from a leade at 1600 f.p.s. and above. Here we have a bullet sitting perfectly still and suddenly it is hit in the butt by 30,000 to 60,000 Ibs. of pressure that wants to accelerate to 1600 / 2500 fps. The gas is not at the speed of light but it is pretty darn quick. No bullet sitting in a leade is a perfect fit. If it was it would be as hard to pound out of the leade as it is to pound on down the barrel. Since the bullet is not a perfect gas seal it simply has to leak some before it can start to move.
The bullet is the weakest component compared to the breech, the case and the bar-rel. It has to give but cannot move at the speed of the gas instantly. Gas leaks around the bullet almost certainly eroding and carrying tiny particles of lead with it and down the bore ahead of the bullet. We know that this very thing happens to steel (the source of barrel wear) so it’s nearly impossible to believe it doesn’t happen to lead. We get away with this because the gas check comes along and scrapes the bore clean each and every shot.
The better the bullet fits the leade theless lead is deposited in the bore in front of it all other things equal - and the more accurate the load will be. If the bullet fits badly, enough lead is left in the bore that the gas check runs over it and lead is left behind. Bullets recovered from a sawdust trap with a load that leads show lead on the side-walls of the gas check. Bullets from accurate non-leading build-up loads do not.
This theory assumes that all loads lead and it starts in the leade. In my experience it does, but I have seen bores leaded at the range where only the last 6 inches or so appeared to be leaded so there are probably other factors at work. Most theories of leading assume that lead is rubbed off in the bore by a mechanical action. With a really rough bore this may be true. Leading from bullet blow-by usually occurs in the first six inches of bore and is easily felt with a cleaning patch as a tight spot. This can usu-ally be wiped out with one dry patch, at the most - one wet patch followed by a dry patch. However this does not have to be done, as this is a normal shooting condition. The rifle can be shot indefinitely, without cleaning, up to the accuracy potential of the load and rifle. This ‘tight’ spot cannot be seen from the breech, in fact the bore appears to be perfectly clean. Looking across the muzzle in a strong light the bore appears black. When you run a patch there are no flecks of lead, just a gray paste. Obviously this type of leading is not stuck to the barrel steel (soldered); it’s sort of floating on top. This is the condition that the gas check acting as a scraper can clean out each and every shot.
While I am sure that the check is a scraper, it does not necessarily have to be made of a copper-based alloy nor does it have to be made in a cup shape. Gas checks custom made of aluminum seem to work exactly like regular checks. I have had both Dan Hudson and Larry Jennings check my results in their custom rifles. Checks made in the shape of a ring or washer and swaged on the base of the bullet seemed to work equally well. A boat-tailed bullet of this design was on the cover of TFS #97.
If my analysis of the problem is correct it also explains some rather unusual characteris-tics of paper patched bullets. Col. E. H. Harrison established in the 60’s that a paper patched bullet with a rather soft alloy could be shot at nearly full jacketed bullet velocities with hunting accuracy from a .300 Win. Mag. If alloy strength were as important as some people claim this simply would not be possible. Certainly a tube of paper does not add any great structural strength to a column of lead!What it does do is to keep the lead insulated from the gasses at the time of ignition. It does not stop blow-by but it stops lead from being in the blow-by.
In summary probably all fixed ammo loads leak gas at ignition carrying particles of lead. The gas check acts as a scraper taking most of this out the bore with each shot. If the lead is more than the gas check can handle (bad bullet fit in the leade or excessive pressure) true leading at the fleck or bore coating level results with subsequent inaccuracy. No manner of wad or filler has to date worked as well as a gas check. No other theory that I’m aware of can explain the rather novel properties of the paper patch bullet.
This theory assumes that all loads lead and it starts in the leade. In my experience it does, but I have seen bores leaded at the range where only the last 6 inches or so appeared to be leaded so there are probably other factors at work. Most theories of leading assume that lead is rubbed off in the bore by a mechanical action. With a really rough bore this may be true. Leading from bullet blow-by usually occurs in the first six inches of bore and is easily felt with a cleaning patch as a tight spot. This can usu-ally be wiped out with one dry patch, at the most - one wet patch followed by a dry patch. However this does not have to be done, as this is a normal shooting condition. The rifle can be shot indefinitely, without cleaning, up to the accuracy potential of the load and rifle. This ‘tight’ spot cannot be seen from the breech, in fact the bore appears to be perfectly clean. Looking across the muzzle in a strong light the bore appears black. When you run a patch there are no flecks of lead, just a gray paste. Obviously this type of leading is not stuck to the barrel steel (soldered); it’s sort of floating on top. This is the condition that the gas check acting as a scraper can clean out each and every shot.
While I am sure that the check is a scraper, it does not necessarily have to be made of a copper-based alloy nor does it have to be made in a cup shape. Gas checks custom made of aluminum seem to work exactly like regular checks. I have had both Dan Hudson and Larry Jennings check my results in their custom rifles. Checks made in the shape of a ring or washer and swaged on the base of the bullet seemed to work equally well. A boat-tailed bullet of this design was on the cover of TFS #97.
If my analysis of the problem is correct it also explains some rather unusual characteris-tics of paper patched bullets. Col. E. H. Harrison established in the 60’s that a paper patched bullet with a rather soft alloy could be shot at nearly full jacketed bullet velocities with hunting accuracy from a .300 Win. Mag. If alloy strength were as important as some people claim this simply would not be possible. Certainly a tube of paper does not add any great structural strength to a column of lead!What it does do is to keep the lead insulated from the gasses at the time of ignition. It does not stop blow-by but it stops lead from being in the blow-by.
In summary probably all fixed ammo loads leak gas at ignition carrying particles of lead. The gas check acts as a scraper taking most of this out the bore with each shot. If the lead is more than the gas check can handle (bad bullet fit in the leade or excessive pressure) true leading at the fleck or bore coating level results with subsequent inaccuracy. No manner of wad or filler has to date worked as well as a gas check. No other theory that I’m aware of can explain the rather novel properties of the paper patch bullet.