I have a couple of double rifles, and o/u and a sxs. I have not been able to purchase any good reference books on this subject, so ask the more experienced members here for their collected wisdom.
I need to know the basic principles involved in developing a load that regulates well in both rifles. My Merkel 450-400 shoots OK with Hornady factory loads, 400 grain dangerous game ammo. But I think it could do better. I also have a Beretta O/U in 9.3x74R that has been properly regulated with Norma Oryx 286 gr. factory ammo.

So here goes:
- assuming same bullet weight, what makes each barrel of a sxs double shoot more to one side or the other? What makes the groups converge, cross or diverge? Is my assumption correct that increasing powder charge / velocity makes bullet impact move horizontally towards the same side as the fired barrel? ( to make a right barrel shoot more "right" increase powder charge & velocity??? )

- what effect does a scope have on sxs double rifle regulation? My merkel was regulated at factory with a scope, but scope model / weight was not listed. what should I expect when shooting iron sights at close ( <75 yds) range?

- I assume my O/U will shoot differently with scope attached and not. It has also been regulated with scope. 4 shot composite groups are very good from 100 yds with scope. 2" or so total. What is reasonable to expect for vertical divergence at 50 yards without scope?
Any hints for me while I develop handloads to duplicate the Norma ammunition? I have the correct bullets and brass and a chronograph.


for background info, I shoot the rifles handheld, from standing position with a soft pad midway on the rifle to minimize effect of a rest. I shoot two shots within 5 seconds, R/L or Bottom/Top then wait a few minutes for barrels to cool before shooting another two.

Books by Graeme Wright on the topic, such as the one below, are a great place to start.

"Shooting the British Double Rifle: A Modern Guide for Load Development and Use" Book by Graeme Wright

Also Cal Pappas has written on doubles for some time. He should be along soon.

Two people on this forum forgot more than most of us know about Double Rifles, MacD37, and Cal Pappas. They know Doubles!

Gents:
I will make a copy of this post to use again as I seem to repeat myself over the months and years. And, thanks for the kind words.

When a single barrel rifle is fired the rile recoils back and the muzzles rise.
When a double rifle fires the rifle recoils back, the muzzles but the right barrels moves not only up but up and to the right while the left barrel moves up and to the left; this being so as each barrel is to the right or left of center.

Because of this, the barrels are not soldered parallel but with a bit of convergence. There is no rule of the amount of convergence as too many variables come into play. Some are: barrel length, barrel weight, bullet weight, bullet shape, bullet velocity, amount of bullet that is in contact with the bore, powder weight, powder burn rate, weight of the rifle, length of pull, shape of the stock, thickness of the barrels, and there are more, I’m sure.

That’s why one reason or the price of doubles the the building and regulation process. Also, that’s why doubles are made for one built weight and velocity.

All that said, doubles can be made to shoot accurately with modern powders with formulas and rations to replace cordite or black powder. They have been published here but I have a list if you want to PM me. To begin with and most important, use the same weight and generally the same shape of bullet your double was made for. Second, use a powder and charge that will give you the same velocity. Don’t get carried away with the velocity issue. Remember stated velocity figures from the old days were taken from 28-inch barrels and may have a bit of blue sky added to the figures to promote one cartridge over another. I have found that when I subtract 25 fps for each inch of barrel that is less than 28” that usually works—either right on or in the ball park.

If a double shoots both barrels to one side but in an acceptable group, I would just tap the rear sight in the direction you want to bullets to print.

If the bullets print wide on the target, the velocity is too slow. If they crossfire, they are shooting too fast. Powder adjustment will solve this problem.

Sight adjustment will solve the problem of good groups shooting too high or too low.

Adding a scope changes the weight and therefore the recoil characteristics. If the rifle is re regulated to make it shoot well with the scope then some of the accuracy with open sight will be lost. My solution to this is if a scope is added don’t re regulate the rifle. Rather adjust the scope to shoot accurately with one barrel—I.e. a very accurate single shot. And, if the need arises to use open sight for close work, remove the scope and you have the accurate open sighted double you started with!

I have never shot and over and under double so have so experience, thus no comment, on accuracy after adding a scope. But I would guess the grouping would not change much as there is no right and left issues to deal with.

My doubles all shoot well with loads I’ve developed for them with some being very accurate (1 inch at 50 yards with my .350 no2, to average 2-2 1.2 inches with most, up to three inches or a bit more for the bore rifles (12-10-8-4).

I use IMR 4831 in my large nitro cases, IMR 3031 in the medium and small nitro cases, IMR 4198 in my black powder and light nitro cases, and Blue Dot in the bore rifle cases. Other powders will work also but these work for me so I stopped the experimentation.

If you can't get your double to shoot, you can have it re regulated. The advantage to this is you can have it made to shoot the load of your choice as long as the pressure is within safe levels.

I hope this helps. PM or email me (<pappas@mtaonline.net> if I can be of any service. Also, our Alaska double shoot is the first of May. Please come.
Cheers,
Cal

Thanks Cal, that is exactly the kind of summary from an experienced mentor that I was hoping to receive. Your explanations make perfect sense, and I greatly appreciate the advice.
However I am puzzled a little by the following: "If the bullets print wide on the target, the velocity is too slow. If they crossfire, they are shooting too fast. Powder adjustment will solve this problem."
In my feeble attempt to understand the physics involved, I would have made an error and concluded the opposite. Faster bullet, & heavier powder charge = more recoil = more recoil movement toward the same side as the fired barrel. Or does the effect have more to do with shorter dwell time in the barrel so that recoil does not have as much time to move things laterally with a faster load? I believe the advice, I just don't understand why.

If you do a search here you will find all this explained. Regulation of DRs is due to muzzle flip and jump; which is why a slow bullet is thrown wide and a faster bullet might cross. They leave the muzzle at different times in the recoil cycle.
I have built several double rifles and the barrels can't be parallel.
Also, it is not just the velocity and bullet weight that is important; it is HOW the bullet gets to that velocity that is equally important. IE, if you load the same bullet to the same velocity but use two different powders, they will not regulate to the same place.
So, even though you have the correct velocity, it still might not regulate because the powder weight and acceleration rate both contribute to when the bullet leaves the muzzle in the recoil cycle; that is vitally important.
O/Us? Those are not really double rifles; just rifles with two barrels about which I know nothing.

Imagine the right barrel firing in very slow motion.
The striker hits the primer and the powder begins to burn. The bullet is traveling down the barrel. The muzzle begins to rise in recoil as well as move to the right of the center axis.
The bullet exits the barrel but the velocity is a bit too slow and the bullet hits the target three inches to the right of the target’s center.

You add 2 grains of powder to increase the velocity a bit.

The bullet now exits the barrel a fraction of a second sooner so the barrel has not moved to the right quite as much. This causes the bullet to move a bit to the left on the target as it exits the muzzle just a bit sooner as the barrel is moving to the right in recoil, thereby bringing the bullet closer to the target’s center.

Add too much powder and the bullet exits even earlier as the muzzles rise and move to the right so the bullet will print more to the left. Remember the muzzles are not parallel but converge a few thousandths of an inch.

Think of it this way, theoretically, if there was no recoil and the barrels converged the right barrel would shoot to the left always and the left barrel would shoot to the right always. This does not happen due to the barrels (think muzzle) moving up and to the right (or left) during recoil.

Does this make sense?
We are only talking about a small amount of powder, 1-3 grains usually.
I hope this helps. If not, ask away.
Cheers,
Cal

PS. dpcd's post hit as I was typing mine. Between the two your questions should be answered. I do not have any experience in building rifles. I don't even turn the screws of my doubles so dpcd's experience is far greater than mine.

Here is one example I had recently which will give you an idea of what happens; a new 450 NE; It came with regulation load data of 98 grains of 4831. Now, I, thinking I would lessen the amount of powder I had to use, loaded some rounds with 4350, about ten grains less, but at the same velocity. Results were what I should have known would happen; the shots crossed 4 inches at 50 yards. When I then loaded the right laid of 4831, presto, it suddenly jumped right back into 1.5 inch R&L group at 50 yards. The bullets left the muzzle at a different point in the recoil cycle. Same velocity; so powder amount and burning rate matters more than you might think. Except in high velocity rounds, it doesn't matter as much.
OTOH you can sometimes use the 2/3 rule and lower the bullet weight or powder and they will still regulate. Sorcery.
Double rifles are fun, a black art rather than a hard science, and sometimes, frustrating. But they all run on some basic rules. Mostly.

Cal and DPCD,
As usual, you do not disappoint!

Thanks; you realize that I make most of this stuff up as I go.

dpcd beat me to it.
That was my line.
C

quote:

Originally posted by mbogobutch:
Two people on this forum forgot more than most of us know about Double Rifles, MacD37, and Cal Pappas. They know Doubles!

mbogobutch Thank you for the kind words, but I think Cal Pappas is the top of the line in the double rifle field. You would do well to buy all his books. There is a ton of information in that library, that will cut years off your learning curve!

………………………………………………….. Enjoy your double rifle! It is the only true big game hunting rifle system that was designed primarily for hunting, while most other firearm types started life as a weapon of war!

MAC:
Thanks for the kidn words but it is I who bow to your wisdom, age, and experience.
Cal

thanks to all who responded to my original post, this has been very helpful.