A friend of mine met Bill Bartrum at a gun show in Spokane, WA. My friend told me that since I was interested in learning about bench rifles & accuracy I should give Bill a call. I did and we had quite a few interesting phone calls. This was about 2005.
Bill was working on a project and at the risk of being a little long, I would like to post one of Bill's emails.
Our latest project is trying to obtain 3,500 fps with a 220 gr bullet in a 308 caliber. the barrel features a left hand side, & a right hand side chamber 2 inches in front of the receiver chamber. Using 45 gr of AA 2520 powder in the chamber, & 40 gr in each side chamber we are obtaining somewhat over 2,800 fps with only a 20 1/2 inch barrel. With 125 gr's of powder & that short barrel, you won't believe the ammount of flame we are getting. We will build about a 34 in barrel soon to see if the 3,500 fps can be safely reached. If this project works out, we will move forward to a 50 caliber, if we can obtain an increase of about 750 to 800 fps in a 50 cal, it will make it a much more effective sniper rifle, outperforming any of their new 416 calibers. We really like thinking outside of the box. The thinking with these side chambers relates to this- although a 308 might hane about 62,000 psi chamber pressure, at the muzzle it has reduced to about 7,000psi. Our computer modeling shows if we can maintain 40,000 psi all the way through the barrel the desired velocity can be obtained. Will it be loud? Oh yeah! It is now with the short barrel...Bill
Kurt,
The two side chambers are set at 90 degrees to the barrel, with a hole drilled through the sides of the barrel. For this first protype rifle the opposing 90 degree side chambers were considerably easier to machine, although we know ( from hydraulics designing) that side chambers angled at about 45 degrees facing the front, would be more efficient.
The receiver cartridge burns powder from the back of the case to the front, however when the fire & pressure passes the side holes the flames ignite both side chambers at the same time. However now the powder is burning from the front of the case to the rear, which builds pressure at a slower rate. With about 60,000 pounds of receiver chamber pressure, at about 2 inches in front of the bullet the pressure has dropped to about 40,000 pounds.
The idea of the side chambers is to maintain 40,000 pounds of pressure completely through the barrel, behind the bullet (instead of dropping off to the normal 7,000 pounds at the muzzle), that's where the additional velocity will come from.
The first cases for the side chambers were left as they come from the factory- we soon learned those necks collapsed after firing, after cutting the necks off we learned the shoulders then collapsed- so we cut te shoulders off & with a straight case it works very well.
The side chamber case is inserted with only the base sticking out, a thick cap is fitted with a recess the depth of the protruding case base, is bolted with four 7/16th bolts ( slow to load but solid).
We had no idea what to expect the first time out, so we hooked up a electric operated relay to trip the trigger, a long set of wires attrached allowed us to get inside our railroad car we have for storage at the range, before touching each end of the wires to the battery as we really had no idea of what to expect. Man but that thing was loud & flames were probably about 6 feet wide with this 21 inch barrel.
Even so, this short barrel produced a bit more than 2,700 fps with 220 gr bullets. These results matched our computer predictions very closely, & it predicts the need for a 34 inch barrel with the angled side chambers to obtain the desired 3,500 fps goal.
The two friends I work with, one is a mechanical engineer, the other had a Doctorate degree in math, so we are not really working entirely in the blind. We just really enjoy experimenting in "gun" areas where we don't believe anyone else has approached in the same way. I can send you a photo if you would like to see this return to battery rifle.
I just cannot agree with anyone stating one product that has been in business for a long time makes junk. If so, they would soon be out of business. We have found that not all of the Lapua cases are the same, believing they most likely "farm out" some of the manufacturing of brass elsewhere. In our testing we noticed thatin some rifles Norma brass is as good as the Lapua, but not in all rifles. Reason unknown for sure. All I can say is if it works well, use it. To heck with opinions of people who have not seen the results you are obtaining. I shot many under .100, 5 shot groups at 100 yards in my 308 HBR rifle with Winchester brass. The one best one meaured out at .009...in that particular rifle I have no problem using Winchester beass. Other rifles prefer Lapua so I use Lapua in those rifles.
Keep me posted on your results with the Norma brass in your 222, that is a fine cartridge. I have a group rifle built in 1953 chambered in 222 ( made by Allen Bench), no idea how
