6mm/8mm Multi-shot shotgun chronoing conundrum

[Coz]

So I will be on the lookout for a Gas M500 very soon and one thing that is getting to me is the whole 8mm vs 6mm and how to chrono a multi-shot weapon.

I did ask on the FB page of the site I will be using it at and the response I got was we will chrono it with one BB and see how powerful it is, This really doesn't help as it will obviously be more powerful firing 1BB than 3 8mm BB's or 5 6mm BB's

It is mostly the 8mm one that interests me as its something different (I don't own any 8mm guns) It is also the 8mm one that is twisting my melon due to the whole weight of the bb and do you chrono with 1 BB and then divide it by 3?

If anyone can shed some light on this (Preferably someone that uses an 8mm gas mossberg )

I don't want to buy an 8mm version and then be told I can't use it because its too powerful.

Cheers

[Tom Andrews]

Re: 6mm/8mm Multi-shot shotgun chronoing conundrum

You might be able to use the chronos that rely on shooting above the chrono between splayed guide posts. They are used for archery and paintball.

[TriggerHappy]

Re: 6mm/8mm Multi-shot shotgun chronoing conundrum

Unfortunately FPS isn't linear, in a perfect world a .10 gram bb would be propelled twice as fast as a .20 gram bb, but it just doesn't work that way. So shooting one and dividing by 3 or however many bbs you're shooting won't work.

[bobfish]

Re: 6mm/8mm Multi-shot shotgun chronoing conundrum

as mr trigger happy says the rlationship between mass and velocity is not liniar.

1/2mv^2

anyway...
I was having a similar thought regarding 8mm bbs.

chornos work by the bb breaking two beams (or by the time taken to break a single beam) So if your 6mm bb takes X time an 8mm bb will take longer because it's 2mm bigger.

so with an 8mm bb, you can have it hugely over the limit because the chronos cannot test it! D:

[Trigati]

If its measuring time across 2 points it may work. The leading edge of the BB would break the first beam, starting the counter and then the leading edge breaks the second beam stopping it. The actual size or mass of the BB does affect this. However I've no idea how the actual chrono units work and how sensitive they are and if the extra 2mm passing through the beams will affect the timing.

Regardless of size, if equal mass and equal speed then the energy is the same. We tend to measure energy at the muzzle so can ignore wind resistance etc. Another thing that's difficult to account for is the internal efficiency of a device if its firing multiple projectiles.

One way to account for that is to use an old fashioned method of impact comparison....

Get some soft clay, make if a flat surface of sufficient thickness. Fire a known single shot into it, say an AEG at 300fps. Measure far it penetrates. Then fire the three shot device into the clay and measure all three. If both guns were the same power you'd expect the results to be close. In this example the AEG penetrates 30mm and each of the shots from the tri-shotgun penetrates about 10mm

You could actually work out the formula to convert penetration to fps for easy comparison. If the above example each of the tri-shot was 15mm deep then total would be 45mm. So for AEG 300fps/30mm =10fps per mm. As the other gun got 45mm it's 45mm times 10fps per mm therefore 450fps equivalent.

There are many other variables to consider, and many assumptions I've not listed but its a cool exercise in physics to work it out....

[Binngoo]

Re: 6mm/8mm Multi-shot shotgun chronoing conundrum

Some chrono's can adjust their calculations according to weight so you could just set it to whatever the weight of the 8mm bb is and see if you get a sensible reading.

For any multi shot shotgun you should chrono it on 1 bb, while this will often be more powerful it needs to be this way. if you shooting 320fps with either 3/5bbs that's fine. But if you missloaded or the gun missfeeds and you accidentally shoot someone with just 1bb but that bb is doing 500fps (probably not possible but its an example) then that person is not going to be happy. And I doubt you wouldn't appreciate it if you were on the receiving end of that!

basically chrono shotguns with just 1 round as thats the maximum velocity they can possibly produce

[loki7491]

Re: 6mm/8mm Multi-shot shotgun chronoing conundrum

A connundrum i am facing myself for an ongoing project!

We have taken the "less than scientific, but very accurate" shoot them in the arse cheek approach.

Take a number of weapons known to be below site limits.
Measure distance from muzzle to bb hitting floor with NO HOP APPLIED
Do the same with shot gun - this will only give a rough indication.

Then take one poor sod who lost a bet and place him down range.
Shoot him with a number of different guns and compare the pain factor!

Is it perfect? Good God no. But if my ammo is falling short of AEGs and i can shoot by buddy in the backside with it at point blank without the requirment for hospital, it is safe to say we are in the right area in terms of muzzle energy.

Will your site accept that? You'll have to let head marshal shoot you point blank to find out i guess (or let you shoot him)