Building a chrono light box

[memilanuk]

Not sure if this belongs here, or in the Reloading forum, or somewhere else entirely...

Recently acquired a NIB Oehler 35P with a 2 foot light bar, with the intention of putting together a 4 or 6 foot setup with a dedicated box housing artificial lights, etc. that I can mount on a heavy-duty (surveyor) tripod.

I know I'm not the first person to do this sort of thing... but I'm finding surprisingly little useful material while searching Google, YouTube, etc. Anyone here know of some good examples related to what I'm looking to do?

Thanks,

Monte

 

[alinwa]

Monte, I've done this a bunch.

I own a construction company and have 10-12, maybe even 15 construction-grade tripods and don't think you'll be able to make any sort of a tripod setup 4'-6' long.

#1 problem is weight of the cover apparatus but #2, and a bigger problem is wind. Anything that large will present too much area to the wind. My gut says plan on two guys, two sawhorses and a plank-mounted setup......"sawhorses" to be the little metal sawhorse clamps and bring along a few studs and a skilsaw, set your ht/pitch at the range.


opinionby

al

 

[memilanuk]

Hey Al,

Long time no see

Bummer to hear that those construction tripods won't hold the weight... I'd planned on keeping things pretty light... had in mind something along the lines of a torsion box base, probably built using 1/4" ply, skeletonized as needed, skeletonized side panels / top, both to cut the weight and the surface area for wind. FWIW, what I was thinking was *two* surveyor tripods, not just one, since most of the setups I've seen for even stock 35Ps at 4' or more use two (light weight) tripods.

I'll have to take another look at the topography in front of the firing line... at the one range, the ground drops off several feet in front of the line. The others... might be more or less level, but are in bays that only allow 100yds.

Something someone suggested elsewhere that sounds interesting, but still might need some 'engineering' to make it work... putting the rail inside large diameter Sonotube. Wind would still be a problem, though.

I've found various info on the 'Net where people used battery-powered LED light pucks or sticks for illumination.

The info is coming together slowly, but it'll likely be after FCNC @ the end of October before I have time to start putting pieces together.

Monte

 

[lpreddick]

I had problems getting my chrony to see .20 cal bullets and was advised to put puck lights over each diffuser. These light are intense! I made a collar device to get the pucks about 1 " off the diffuser so the light would be uniform. Painted the bullet's ogive black and I get nearly 100% reads.

 

[Ringostar]

Monte, Is it practical to do your testing from the ground like when we shoot competition? Your whole box could lay on the ground in front of the rifle. Just an idea that I've thought about if I can find a good used Oehler at a decent price.

 

[memilanuk]

Yes, but... where I typically practice at, the ground drops away in front of the firing line. Even shooting from prone, the chrono is about 3 feet off the ground.

 

[jlow]

The thing to keep in mind is as you add longer rail i.e. 4 to 6 foot, the whole thing gets more heavy, unwieldy and more difficult to transport.

I have modified my 2’ rail to 4’ and that just fits in my truck – that to me was worth it. I think the 6’ is going to provide challenges. Both the 6’ rail and light bar adds complexity and the first question you should ask yourself is “is it necessary and do I need it?”

It’s a bit like reloading, you can start off by putting everything you’ve ever heard about getting precision and you will quickly find the amount of work and time spent to load precision rounds pretty impossible and you are spending most of your time reloading vs. shooting. What is best if to only do the stuff that makes a difference… I would suggest you try going to a 4 foot bar and without the lights and see how that goes first.

 

[memilanuk]

Good point about the portability of the setup. I'd picked up a 10 foot stick of 1/2" conduit at the local hardware store, and had started thinking about going with an 8 foot setup instead of a 4. Then I got to thinking about getting the blasted thing to and from the range... I've got a short (6.5 foot) bed GMC currently, and a piece of 8' conduit would just barely fit diagonally. Screens would have to come off, etc. adding to the PITA factor during setup. 4' should fit under the tonneau cover length-wise easily enough, as should (I think?) a 6 foot.

The first couple range sessions just 'as-is', with the 2 foot bar, fell under 'okay' and 'WTF?'. I think the spacing is a little off - the first time around one channel read considerably higher than the other. Not enough to trigger any warnings, but enough that it made me wonder a bit. The second trip... must have gotten things flipped around the other way and/or something as the readings were *way* off, and several of them tripped the error warning. Maybe once I get the thing semi-permanently mounted to a beam and the screen spacing fixed it will be less of an issue. I had considered making an adjuster block to clamp on the rail to nudge the screen just slightly, and in a controlled fashion.

Right now its load-n-shoot mode, though. Load for the Northern Rockies F-class LR Regional @ Deep Creek next weekend, then load for the Rattlesnake October LR Regional, then load for the USA FCNC. Then I get a few months off before the BSWN

 

[jlow]

Generally speaking, longer give better accuracy but the real question is what is your required accuracy? If you have the 35P, look in the manual at the very back. You will see that the MVs we mostly deal with which is around 3,000 fps, a 4’ rail will give you a variance of around 4 fps. Going even to an 8’ rail will only bring that down to 2 fps. The question is in terms of the hassle of a 6’ or 8’ rail you have to deal with, is it worth the 1-2 fps difference? We are not Brian Litz... Remember it is not just whether it fits in your truck but also the chance of damage increase as it gets longer.

I’ve used mine for the last couple of years a lot - weekly. One thing most people complain about the Ohler is the hassle it is to use – that is generally true. The way I find best to deal with it is the less setup you have to do, the less hassle it is. So for example, you never want to take the screens off because the exact distance between the screens is critical to get an accurate reading because the machine assumes it is in my case exactly 2’ between screens. This is one of the hardest part in terms of setting up a different rail. It is of course hard to get it back to the same position (location and angle) if you take it off and then put it back on – so you never want to do this. Mine is actually drilled through the conduit from the bottom. So if you look at this photo of mine, you will see the set screw on the bottom of the screen. That screw is not just a set screw to hold down the rail, but it actually goes completely through the conduit and then pushes up against the bottom of the detector. That way it cannot move laterally nor can it rotate.

Also take a look at the cords, they are bundled together and zip tied to the rail. At the end where it plugs into the display, the cords are color coded so that they plug into color coded female ends. So for me, there is also a quick release that is permanently attached to the bottom of the rail. That snaps into its partner on the tripod – again fast.

The last thing that is a big hassle is lining up the screen to the muzzle. This turns out to be super easy. Put your gun up at the bench and get it lined up to the target (I use a rest). Make sure it is unloaded. Take the detector tripod out and put it in front of the rifle. Put your eye right where that yellow square (ignore that) is in the front detector. Look down the rail and through the scope. When you position the rail so that you can see clearly through the scope, you are centered. I then further test with a laser bore sighter and it is perfect 95% of the time.

 

[memilanuk]

All true... most of the things you mention touch on things that I have planned - bundling the cords, color-coding the plugs, quick-release shoe for the rail-tripod connection. I may not have quite the same requirements as Litz, but I do have some projects in mind where the more instrumental accuracy, the better. For routine 'day-to-day' use I still love the convenience and 'good enough' accuracy of my MagnetoSpeed.

A couple questions, while I've got your attention...

1) 4 foot bar with only a center mount - no issues with sagging?

2) In the pic, it looks like there is something in the first and last screen - a clear sheet with an aiming point for the laser, perhaps? Any details on that? I had thought of doing something similar but with a grid pattern.

 

[jlow]

Sagging is not a problem but here you need to make sure you find a straight conduit (of course this assumes you are not going to add more weight i.e. lights to this...). This was a concern of mine when I build mine. I thought well if I buy the rail from Ohler, that would give me the best quality. This turned out to be wrong. The reason is Ohler basically use the same conduit that you and I buy but with one difference. They dimple the conduit at the location where they think the detectors should be. The problem with this is dimpling the conduit cause a slight bent in it….. : So I found that it was better to drill through instead of dimpling.

Yes – good eyes, that was a project of mine to get exact location for where the bullet will go through. I found it was not necessary.

In terms of accuracy, here is something to ponder. Yes, we would all love to have 1-2 fps accuracy, BUT lets take a quick look at how accurate the distance between detectors will affect this. For a 4’ rail, if we divide it into 3,000 units (remember this is the approximate fps we are interested in), a difference of 16 thousands in terms of distance between the detectors will change our fps by 1 fps. The question is can you position/drill/dimple to that degree of accuracy?