Drilling Blind Hole in Barrel Worst Case Scenario?

BullOnParade

BullOnParade

CGN Regular
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Eastern Ontario
So I've been slowly working towards making my Mares Leg a faster camp gun, easier to use for myself and my (less practiced) wife. It's light and short enough to holster carry and that means it won't be leaning against a tree when I need it.

The most important project is getting a red dot sight. They're faster than traditional sights and better in low light scenarios. Last spring I machined a small section of rail with a dovetail similar to what's available on the Skinner sight website. The proof of concept was great, but I wanted to try something else.

I've machined a longer piece of rail to screw to the top of the barrel, and today is the day I drill and tap the barrel for final fitting. I'm sweating bullets, figuratively.

What's the worst case scenario if I drilled through the barrel? Is the barrel pooched? If the hole were threaded and filled, will the barrel function? I'm double checking my math and taking extra care in my setups, but I figured I'd ask the CGN masters their opinion.
 
Does a gas port ruin a barrel?
Drilling through into the bore is sloppy work, but the barrel would still be usable with the hole tapped and plugged.
Crooked, misaligned, off-centre holes are worse.
How are you locating the holes?
How are you controlling the depth?
What equipment are you using?
Drilling and tapping a row of properly located holes (top dead center and properly spaced) on a round surface is an interesting undertaking. Done carefully with proper equipment, it is still a bit stressful.
I use a Forster jig that makes the job relatively straightforward.
 
tiriaq said:
Does a gas port ruin a barrel?
Drilling through into the bore is sloppy work, but the barrel would still be usable with the hole tapped and plugged.
Crooked, misaligned, off-centre holes are worse.
How are you locating the holes?
How are you controlling the depth?
What equipment are you using?
Drilling and tapping a row of properly located holes (top dead center and properly spaced) on a round surface is an interesting undertaking. Done carefully with proper equipment, it is still a bit stressful.
I use a Forster jig that makes the job relatively straightforward.
Click to expand...

I'm a tool and die maker by trade working in a small stamping shop with good, manual equipment (no CNC). The barrel will be indicated on the mill for level and square and the taper has been measured several times on the shadowgraph. The barrel is octagonal, which is a gift and a curse with flat sides and constant-ish taper (each of the four sets of sides measure approximately 0.21°, but I'm only particularly interested in the top and bottom flats, any inconsistency in the sides will be split between the two faces when indicating in the vice).

To drill perpendicular to the bore and resist pulling, I will mark the hole first with a ball nosed endmill before the center drill. The threads will be #6-40, because I have a tap (and spare) from working on my 870.
 
You should be good to go. Most bases use 6-48, but 6-40 would work. Only advantage to -48 is that more threads can be cut in a shallow hole.
 
If drilling the barrel worries you there is another remedy and I just completed doing 5 of my very old lever-guns (100 - 125 yrs old) to accommodate red dots without harming or altering them in any way that would reduce the "collector value.

From your post I assume you have access to some machining tooling? I just built a mount bracket that attaches to the screw holes already in the tang to accommodate Marbles type sights. Some newer guns don't have these screw holes from the factory but are easily drilled & taped safely with out taking the chance of drilling a barrel right through.

I purchased a bar of "Picatiny Rail" and cut it into 2" pieces, just enough to hold a red dot sight base. The only other components are a couple pieces of 1/8 thick aluminum andle, one about 4 or 5 inches long and another the same length as the picatiny rail and a couple hardare machine screws . Other than that and a little ingenuity and trial & error for the first one. After the first one & and all the trials & tribulations that come along with proto-typing, it now takes about 2-3 htrs with a milling machine & drill press to complete one.

One thing you will notice with a red dot on a lever gun is that the stocks aren't built for easy use of any optics mounted on top. Even if Barrel mounted ,the base required (Picatiny or otherwise) along with the adjusting base on the actual sight make them high enough that an ordinary solid cheek weld isn't possible. My sights are mounted as close to the receiver as I can put them without interfering with the hammer or bolt function.

They aren't fancy and I'm sure would elicit cries if derision from lever gun purist but failing eye sight make them a necessity to me if I want to shoot my old guns.

Pm me a textable phone # & I will shoot you a couple of photo's.
 
I have a dial gauge set up on my drill press... I touch the tip of the drill to the surface of the barrel, set the gauge at zero, drill in the appropriate amount. Leave at least 25 thou material at the bottom of the hole. It is so much nicer to be careful than to drill through...
 
Here's the rail I made,
N5Ny7k6.jpg


I hope to use the end closest to the receiver as a fixed rear sight
6eQIveg.jpg


Clearance slot through the center of the rest of the rail
be0CWSh.jpg



4rwAq4r.jpg
 
The trick is to ideally drill a flat-bottomed hole to allow the screw to grip as far down as possible. I'm in the same situation with installing a front sight on one of my rifles. Modifying the flutes of a #28 drill bit to drill the last bit of the hole flat is pretty difficult. I'll be using some Loctite 380 (black, rubber-modified adhesive as well so the three or so threads the screw will have to grip won't be as much of a problem.
 
A flat bottomed hole is preferred when depth is at a minimum.

I much prefer the 6 x 48 thread over the 8 x 40.

I ground a #31 drill flat on the end and then lightly ground very little clearance. The flat bottom pretty well stops after it takes out what the pointed drill left...
 
guntech said:
A flat bottomed hole is preferred when depth is at a minimum.

I much prefer the 6 x 48 thread over the 8 x 40.

I ground a #31 drill flat on the end and then lightly ground very little clearance. The flat bottom pretty well stops after it takes out what the pointed drill left...
Click to expand...

Thanks, Dennis. Agreed about 6-48 but the FS I have takes an 8-40 screw so I'm stuck with that. I've tried modifying a #28 but it isn't easy (at least for me). I did a trial hole in a piece of bar and it's "good enough for gov't wotk" and will probably do for my project.
 
Drill the hole with the pointed drill first... then insert the flat bottom into the hole before turning the press on... the hole guides the drill better that way...
 
For the record a flat bottom hole doesn't gain anything without the appropriate tap or set of taps.

Project was postponed due to workload on the machine at the end of the day. Hopefully before the weekend.
 
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