Senriku is a rifle with an extra-long barrel with a widely-flared muzzle, similar to a blunderbuss but highly exaggerated. The end would normally render the cross-hairs placed just behind it useless, though that does not seem to be the case.
Senriku is capable of shooting down targets from an extremely long range. Combined with Augur's eyesight, he can hit a gull flying so far away from Jaya, his location, that the island cannot be seen from the gull's position. It can also be used for point-blank range shootings just as effectively, as Augur did so in a collaborative attack against Whitebeard. Also, Augur is capable of firing rapidly, as demonstrated with Portgas D. Ace as the target.
In the anime, Augur was accurate and precise enough to shoot through his opposition's bullets and gun barrels, which easily destroyed both ammunition and firearms per shot. In the ninth movie, Augur was seen firing explosive rounds with this weapon during the attack on Drum Island.