Dan, thanks for your insights, enjoyed reading them.
As you know, things are a bit different here. My need (want) is for something that would be appropriate for poking in and out of holes made by boulders and weird cement pier things (see Manasquan inlet, NJ, USA) looking for a fish or two for dinner. That and a small bag of clams, I'm as happy as I get. No sharks, not even inshore. Only ever saw a few. All offshore, lots there. The water here is nutrient rich, green, and chock full of small to middle sized stuff. I don't need anything more than that.
This is something you should consider... a typical euro gun has a flopper shaft and the line is attached through a small hole at the very rear of the shaft. These two attributes are very important, yet it may not be so obvious to the casual observer.
First the floppered shaft. This means that there is no screw on tip. The Biller gun (with a line slider/line stop) uses a threaded tip. This is a definite weakness at the threaded end. If you blast point blank into something hard like steel or granite of a jetty, you are quite likely to snap the base of the threaded end of the shaft off. This ruins the shaft and often the tip if the threaded end is buried into the tip.
A floppered shaft is stronger because the flopper is attached by a pin via a hole drilled in the shaft itself. Elimination of the threaded end makes for a more abuse resistant shaft. Also, the flopper will make a smaller hole in the fish than a threaded tip which has a larger diameter than the shaft. A smaller hole means better penetration and less chance for the tip to pull out.
So for shooting in holes, this type of floppered shaft is better. You will need to sharpen it with a file or on some concrete after blasting rocks, but it is tough to destroy them.
Also, the line attachment method: Biller versus a euro gun... In my opinion, this is a VERY significant advantage provided by the euro shaft. The line attachment at the rear hole allows the line to trail the shaft and provides less drag and less sideways pull on the shaft than a slider which routes the line at a 90 degree angle to the shaft. However, for shooting in a hole, accuracy and a little drag is NOT really an issue.
What is an issue for hole hunting and especially when shooting into wrecks is the shaft retrieval problem. The Biller gun or any gun which uses a slider is VERY easy to get caught in the hole making the shaft unretrievable. The reason for this is that after the shot, the fish will run back in the hole or struggle. Your goal after shooting the shaft in the hole is to IMMEDIATELY throw the gun backwards under your elbow and away from the hole, then grab the shooting line and pull hand over hand as fast as possible. You want to YANK the fish out super fast before he has a chance to run behind some kind of obstruction. If he gets the shaft behind an obstruction and you pull on the shaft with a slider, the shaft will turn SIDEWAYS.
This turn to a sideways position is caused by the slider pulling on the shaft from a position about 6 inches from the rear of the shaft (where the slider stop is located). This causes the shaft to turn and now you are pulling on the shaft in a T- position. This is how a slip tip deploys in a fish and the absolute wrong thing to happen in a hole. You can not push with a string and if you can not take your tank off and crawl in and reach the shaft and move it with your hand, then the shaft and fish are lost.
However, with a euro shaft, things are MUCH different. You follow the same procedure: shoot, throw gun and yank on line ASAP, BUT with the line attached to the REAR of the shaft, there is NO opportunity for the shaft to "toggle" or by pulled like a T... and be jammed. You can just pull the shaft backwards in the exact same tragectory which it went into the hole. When you are shooting in a wreck with complex pieces of steel bars and debris hanging around, the benefit of being able to pull the shaft out in the direction it went in allows you to get the shaft out ALMOST every time.
There are so many shots I would not take with a gun with a slider, that I now take with a euro shaft. I used shafts with sliders for about 25 years before I saw the light.
A 60 or 70 cm euro gun is what I would use for shooting into holes with low visibility. You can probably get by with a single band.