The only squib I have ever had was with a friend's Browning replica of a 1886 in 45-70 shooting his reloads. He was using a small amount of smokeless powder in that huge case with some sort of filler to hold it next to the primer.../
/...When I shot trap one of the guys was notorious for occasionally forgetting to add powder. The squib I had sounded exactly like his primer only shells. I always worried that one of his no powder reloads would be followed by a double charge but it never happened.
Stories like this cause me to worry. I've been handloading since I was 12 - 44 years and counting.
At least half of those years have involved loading upwards of 20,000 rounds per year. I've loaded everything from .32 ACP and .22 Hornet to .45-70, including cast bullets in numerous rifles and pistols and both black powder loads and reduced power loads using Unique in my .45-70 Sharps, .30-30 1885 High Wall, and Model 94 in .38-55. I've also done some slightly off the map things like developing handloads for the distinctly different 1895 Nagant.
In that 44 years with that wide range of reloading activity, I've had exactly 1 squib, and that was with a federal factory 9mm load that had been submerged in a leaky ammo can during hurricane Irene.
In the past, I rarely heard about squibs, but the last several years, I've heard things like the post above "if you shoot long enough, you'll have one", which is true in the same sense that if you put an infinite number of monkeys in a room with type writers for an infinite period of time, they'll randomly type the works of Shakespeare. Unfortunately, I am seeing way too many handloaders regarding a squib as inevitable.
The fact is that a squib in a handload is 100% due to poor practices by the handloader and if he or she is getting squibs, he or she needs to either get his or her act together, or find another hobby that doesn't require attention to detail.
Similarly, if you are buying commercial ammo and are getting squibs, you need to re-consider the ammo you are buying and buy better quality ammo.
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As for my 9mm flooded factory 9mm ammo squib, I noted the lack of recoil immediately, and upon investigation noted the 115 gr FMJ poking just out the end of the barrel. I'll see if I can find the picture...
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As to using Unique in a .45-70, I get excellent accuracy (1 MOA 5 shot 100 yard groups with commercial cast 405 gr RNFP bullets, with black powder velocities but without the black powder mess. It makes a superb short range load in my Sharps for plinking at plates out to 300 yards or so.
However, I use a 1"x1"x1/4" polyfiber wad (cut from 1/4" polyester quilt batting) stuffed in the case on top of the powder with the eraser end of a pencil. Don't quote me on it, but IIRC the load is about 14 grains - a very small charge in a .45-70 case.
If you forget that wad, one of two bad things is going to potentially happen:
1) a squib with the powder sitting in the front end of the case failing to properly ignite, or worse,
2) an excessive pressure condition due to the powder sitting in the rear of the case, being blown forward and scattered and igniting much faster than intended.
Those kinds of handloading activities are in the big league and you need to both know what you are doing, and pay very close attention to what you are doing.