Rubber components such as vacuum seals are usually coated with a mould release agent that was used in the moulding die that created them or some other material which was later applied to stop them sticking together when they were all grouped together in a storage bin, thus initially they have a slightly lubricated smooth surface. That surface lubricating layer will not last long with physical abrasion as any stepped hard metal edge moves through that seal, such as we have with the stop diameter on a spear tail moving through a vacuum barrel seal of the type that is imprisoned in the muzzle body.
Now rubber components that have been submerged for a long time develop a continuous water film over their surfaces as any one who has used rubber dive fins will have noticed when they first clamber out of the water after a long dive and before the fins have had a chance to dry off. In fact old fins can almost look brand new with an unbroken slick, shiny finish when in reality they are rather scuffed up and dull looking when dry and they will only have water beading on their surface after just a short dunk which will fail to fully coat them. What had happened after the long immersion was that the rubber surface had become fully wetted and thus lubricated even with the fins then lifted up out of the water.
The significance of this wetting phenomenon is that if you muzzle load a vacuum barrel gun out of the water, say to judge the loading effort without actually cocking the gun, then a quick dunk of the muzzle to wet the seal before any diving has commenced may be insufficient to actually wet the seal, while doing this after a long period of immersion will not pose a problem as by then the rubber seal surface will be fully coated with water. To get around this problem you need to provide some initial lubrication in the form of oil drops or a very thin smear of grease that is virtually wiped off again as grease will otherwise hold onto any small particles floating in the water such as silt or sand and you don't want to create an abrasive paste at the entrance to your gun! Successive shots will remove this added lubricant, but by then water will have taken over and the lubricant will only need to be added after the gun has fully dried off again and before the next spear insertion in the gun. If you don't ever put the spear in the muzzle until the muzzle is submerged then you will not run into this problem and I expect that is the case with those who claim you don't need any extra lubricant on the muzzle seal, or the seal is brand new and already has some lubricant on it from the factory, but that will not last very long and over time the seal will get micro scuffs on its rubbing surface (the shaft has a micro texture and is not absolutely smooth even though it appears to be to the naked eye) and either needs a long soak before use or some additional lubricant.
As I don't know the length of the soak time required I think the best bet is to use some extra lubricant which you can think of as a transitioning element, although this will stop the seal surface wetting, but only until the spear shaft passages through the seal wipe it away, or in the case of light oil the lubricant simply floats away in the surrounding water. My guess is vacuum seal damage occurs with the first few shots when the seal is not fully lubricated and the spear has been inserted with the muzzle initially poking above the surface of the water as this action deprives the inner barrel of enough water. The spear needs to be inserted while underwater to allow a plug of trapped water inside the inner barrel to push the seal open as both it and spear tail stop diameter arrive at the seal's inner face during the shot, the volume of the plug of water being slightly greater than the accommodation space inside the muzzle with the spear tail at the minimum insertion of the tail inside the muzzle, i.e. when the sealing lip first contacts the shaft diameter after it drops off the edge of the spear tail stop diameter during muzzle loading. Ideally you want the water plug to arrive first before the edge of the stop diameter does as then the stepped metal edge never actually touches the seal, but if it does then you need the rubber to be well lubricated by being fully wetted. Ideally you want the vacuum seal to momentarily leak water at the very start of muzzle loading (loading being carried out near the surface where the surrounding water pressure is low) and before the developing vacuum in the inner barrel sucks the seal's rubbing edge tightly against the shaft diameter so that the trapped water plug in the inner barrel is large enough in terms of its volume, but only slightly greater than the muzzle accommodation space.
If the shaft tail stop diameter leading edge snags the rubber seal from the rear during the shot and before the water plug does then it will stretch the front section of the seal forwards in the muzzle resulting in tears or cracks in the conical section of the seal, or the cylindrical seal body immediately behind the conical section, which will require the vacuum seal to be replaced.
Now rubber components that have been submerged for a long time develop a continuous water film over their surfaces as any one who has used rubber dive fins will have noticed when they first clamber out of the water after a long dive and before the fins have had a chance to dry off. In fact old fins can almost look brand new with an unbroken slick, shiny finish when in reality they are rather scuffed up and dull looking when dry and they will only have water beading on their surface after just a short dunk which will fail to fully coat them. What had happened after the long immersion was that the rubber surface had become fully wetted and thus lubricated even with the fins then lifted up out of the water.
The significance of this wetting phenomenon is that if you muzzle load a vacuum barrel gun out of the water, say to judge the loading effort without actually cocking the gun, then a quick dunk of the muzzle to wet the seal before any diving has commenced may be insufficient to actually wet the seal, while doing this after a long period of immersion will not pose a problem as by then the rubber seal surface will be fully coated with water. To get around this problem you need to provide some initial lubrication in the form of oil drops or a very thin smear of grease that is virtually wiped off again as grease will otherwise hold onto any small particles floating in the water such as silt or sand and you don't want to create an abrasive paste at the entrance to your gun! Successive shots will remove this added lubricant, but by then water will have taken over and the lubricant will only need to be added after the gun has fully dried off again and before the next spear insertion in the gun. If you don't ever put the spear in the muzzle until the muzzle is submerged then you will not run into this problem and I expect that is the case with those who claim you don't need any extra lubricant on the muzzle seal, or the seal is brand new and already has some lubricant on it from the factory, but that will not last very long and over time the seal will get micro scuffs on its rubbing surface (the shaft has a micro texture and is not absolutely smooth even though it appears to be to the naked eye) and either needs a long soak before use or some additional lubricant.
As I don't know the length of the soak time required I think the best bet is to use some extra lubricant which you can think of as a transitioning element, although this will stop the seal surface wetting, but only until the spear shaft passages through the seal wipe it away, or in the case of light oil the lubricant simply floats away in the surrounding water. My guess is vacuum seal damage occurs with the first few shots when the seal is not fully lubricated and the spear has been inserted with the muzzle initially poking above the surface of the water as this action deprives the inner barrel of enough water. The spear needs to be inserted while underwater to allow a plug of trapped water inside the inner barrel to push the seal open as both it and spear tail stop diameter arrive at the seal's inner face during the shot, the volume of the plug of water being slightly greater than the accommodation space inside the muzzle with the spear tail at the minimum insertion of the tail inside the muzzle, i.e. when the sealing lip first contacts the shaft diameter after it drops off the edge of the spear tail stop diameter during muzzle loading. Ideally you want the water plug to arrive first before the edge of the stop diameter does as then the stepped metal edge never actually touches the seal, but if it does then you need the rubber to be well lubricated by being fully wetted. Ideally you want the vacuum seal to momentarily leak water at the very start of muzzle loading (loading being carried out near the surface where the surrounding water pressure is low) and before the developing vacuum in the inner barrel sucks the seal's rubbing edge tightly against the shaft diameter so that the trapped water plug in the inner barrel is large enough in terms of its volume, but only slightly greater than the muzzle accommodation space.
If the shaft tail stop diameter leading edge snags the rubber seal from the rear during the shot and before the water plug does then it will stretch the front section of the seal forwards in the muzzle resulting in tears or cracks in the conical section of the seal, or the cylindrical seal body immediately behind the conical section, which will require the vacuum seal to be replaced.