Longer Reach

[longcruise]

Good question! My theory is that its the jet of gasses blowing through the shot column as it leaves the muzzle. That would support the theory of large powder charges tending to produce donut holes.

before plastic shot cups came along, modern shells were loaded with fiber wads. Don't recall a donut hole problem with those.

 

[BrownBear]

Then what causes the "donut hole" in a shot pattern :idunno: .

I'm convinced it's more due to gas leakage around/through fiber wads than anything else. Fiber wads are good leakers in my experience, and the issue is largely resolved by adding a tight-fitting hard card or Type A as TOW calls them. I can put one between the powder and fiber wad and eliminate the donut hole. Leave it out, and sometimes you get a donut, and sometimes not. As an added bonus on days with lots of shooting, if your fiber wad is well lubed, the hard card does a nice job of scraping fouling.

As an aside, I think the reason some guys like a stack of OS cards rather than fiber is simply that they're getting a better seal. Haven't tried it yet, but it would be interesting to try putting a few of the OS cards under a felt wad to see if the donut goes away.

No science or high speed photography behind my opinion. Just many years of shooting.

 

[Joel/Calgary]

I'm somewhat surprised at the idea of gas leakage around the O/P wadding causing a doughnut pattern rather than an irregular one, but I can see leakage THROUGH the wadding doing it. Anyway, concerning too-heavy wadding causing doughnuts, two of the possible contributing factors I've seen put forward are:
- If the gas pressure at the muzzle is sufficient, the thick wad column is still being accelerated for that 1/2"-3/4" after the shot has cleared the confines of the barrel, until the rear of the wads clear the muzzle. This would go along with the observation that it's easier to get tighter patterns with longer barrels in the absence of choke, shot cups, etc., than it is in shorter ones.
- Even if the wads are not being significantly accelerated at the muzzle, the powder gasses combined with aerodynamic drafting behind the still-compact shot charge could still cause the wads to bump the rear of the shot charge.
Whichever it is, it's consistent with the spark photographs of pre-shotcup cartridge loads fired from cylinder/unchoked barrels, with the rearmost pellets in the charge spreading laterally very close to the muzzle.

I cannot personally attest to the reasons, but I can to the effect in the 29" barrels of my double, with blown or inconsistent patterns using lube-saturated fibre cushion wads of 1/2" and even 1/4" thickness, and possibly with the 1/4" hard O/P wads combined with cushions. Lighter 1/4"± cushions (the rough bores NEED adequate lube) and substituting multiple O/S cards for the O/P one have made a significant difference in pattern consistency.

Back to powder gas blow-by, my 16ga has somewhat rough bores and I've found I need to use at least 3 and preferably 4 overshot cards to seal reliably (brown burn streaks on recovered paper shot protectors are very distinctive compared to black/grey fouling smears). This is using firm .020-.025" thick commercial or home-made cards at least .015" larger than bore diameter; with thinner materials like milk cartons, it is 4 minimum and 5 preferred. Softer materials like cushion wads, packed tow or bark fiber, etc. appear to need a minimum thickness of something like 1.5x the bore diameter to seal, depending on the material. Materials with intermediate hardness, like hard felt, appear to have thickness requirements in between the two, as one would expect. I haven't seen anyone work out what the minimums are when using combinations of materials to seal over the powder, e.g 1 O/S card + 1/2 of a cushion wad.

Regads,
Joel

 

[Rifleman1776]

Are you talking external or internal?

 

[19 16 6]

Please don't take this personal cannon it's just that to enter the discussion someone has to be replied to.
Doughnut pattern appears on a target pattern sheet shot straight on with a steady hold. A 2D answer. The 3D reality is that the shot column is like a big teardrop shape with most of the pellets flying out the front & tapering back to a few stragglers at the rear. If you keep the muzzle moving, swing with the shot, your pattern is an entirely different thing. Think of the gun working like a garden hose & you are hosing the target down with it. Try it with the hose & you will see the water that leaves the hose later will hit the target area later but also ahead of the initial impact point. This is shot gunning & the whole approach is different to rifle shooting
Soft shot & a big charge will deform every pellet in the load by compressing the column together & the outside ones to the bore thereby deforming them. Deformed shot are flyers & flyers fly outward, hence doughnut. Harder shot & softer charge will tighten the pattern.
Preaching to the choir I expect. :yakyak:
O.

 

[19 16 6]

Sorry I meant Rifleman not Cannon. :redface:
O.